I've been doing some calculations to determine how close my fuel pump is to its flow limit.
I have the Walbro F90000267 in-tank, E85-compatible fuel pump. The official specs from Walbro are as follows:
 
Fuel pressure, Flow rate (at 13.5V)
30psi, 455 litres/hour
35psi, 442 litres/hour
40psi, 430 litres/hour
45psi, 417 litres/hour
50psi, 405 litres/hour
55psi, 393 litres/hour
60psi, 381 litres/hour
65psi, 368 litres/hour
70psi, 355 litres/hour
75psi, 341 litres/hour
80psi, 328 litres/hour
 
Fuel pressure, Flow rate (at 12V)
30psi, 400 litres/hour
35psi, 387 litres/hour
40psi, 375 litres/hour
45psi, 362 litres/hour
50psi, 349 litres/hour
55psi, 336 litres/hour
60psi, 323 litres/hour
65psi, 310 litres/hour
70psi, 298 litres/hour
75psi, 285 litres/hour
80psi, 273 litres/hour
 
EDIT: I already have a separate relay to power the fuel pump directly from the battery, therefore I was assuming that the voltage at the fuel pump would be at least 13.5V, but I later found that it's closer to 13V.
 
I decided to buy and install a fuel pressure sensor from Adaptronic so I can monitor the fuel pressure properly through my ECU. The sensor has a 1/8 NPT thread so I was able to install it directly into the gauge port on my Turbosmart FPR1200 fuel pressure regulator.
After installing the fuel pressure sensor and calibrating it in my ECU, I discovered that my base fuel pressure is 35psi, which is lower than normal, but this is actually good news. Even if my fuel atomisation is suffering a bit, it's been like this all along and the engine performs very smoothly.
 
EDIT: For my own reference, I made a few refinements to the info below since I first posted it.
 
I reviewed some data logs and confirmed that my 386rwkw tune has a maximum injector duty cycle of about 57%.
I have Injector Dynamics ID2000 injectors which flow ~2000cc/minute each at 35psi differential fuel pressure. At 100% duty cycle and accounting for all 4 cylinders, that would be a total of 8 litres/minute.
With an injector duty cycle value of about 57%, my engine would be consuming about 280 litres/hour of fuel at full power.
The fuel pump should be capable of flowing about 330 litres/hour of fuel at 13V and 70psi fuel pressure (35psi base pressure + 30psi of boost + 5psi conservatively estimated pressure drop from pump to rail), which means that it should be able to support just over 400rwkw if I keep the base fuel pressure at about 35-38psi. This is all theoretical at this stage, so I don't know how it will differ in the real-world.
 
I had been considering installing a surge tank with twin staged pumps in parallel to get even more fuel flow, along with the other benefits of a surge tank, but I'd rather avoid installing a surge tank at this stage so it's good to know that I don't really need it. If I ever get better track tyres in the future and increase my cornering G's, only then will I possibly reconsider the surge tank idea. Anyway, if I just make sure that the fuel tank never runs too low at the track then there won't be a problem. 
I'm also giving serious consideration to the idea of having twin staged pumps inside the standard fuel tank.
 
I'm not saying that I'll definitely go for more power either. I'm sure the engine is capable of more, but I hardly NEED more power haha. In any case, it would definitely be activated by a button so I can choose when to punish the engine a bit more than usual (eg. at the drags). 
 

When I had mine tuned I was concerned about the pump as well, but he said as long as its wired up correctly they make just over 400rwkw on e85.

Hate to nit pick with you, but how did you get your value of 35psi? If you did it at key on engine not running, its not a true value. If you did it at idle, it is also not a true value. As you have a vacuum/boost referenced fuel pressure, you will have a lower value at idle, and guessing here, your cams are probably only allowing your car to pull 8psi vacuum (my car pulls about 6.5psi)

Either check at 100kpa absolute, or pull the vacuum hose off, block it off with a bolt temporarily and then start the car and let it settle down to idle and check value.

I regards to the fuel pump you have fitted to the car, i had an opportunity to find the limit of one of these pumps on a 2.9l stroker skyline. With stock wiring, stock lines etc (11.9v to the pump), we topped out at 360awkw on e85 (around 19psi). With a relay and 30amp fuse fitted (yes, they need to be that big, you with pop a 25a fuse on trailing throttle) to get direct battery voltage, we topped out at 410awkw with the boost at 21psi. We then ran duel 3/8 fuel lines, with duel billet fuel filters and each line feeding one end of the rail and topped out the single pump at 450awkw on 24psi.

If you were ever to fit a surge tank, you would need a 5l capacity to minimise boiling fuel. You would also be best off staging the duel pump setup, to minimise fuel heating. E85 is very temp sensitive (think 10-15% change in mixtures when fuel floats between 50-70 degrees, and in a surge tank you will get there.) The worst thing is it is not consistent enough to cover with fuel trims on temp (3-4% not unheard of, which at your power level and boost can cost a motor.)

Just some food for thought, as inline restriction and voltage at the pump play a huuuuuge part on actual flow.

Great info as always, thanks!
 
The base fuel pressure is definitely 35psi. I watched the fuel pressure in the ECU software as the fuel pump primed with the engine off, and it quickly reached 35psi then stopped there. Also, I took a data log during a drive after installing the sensor and the differential fuel pressure was consistently 35psi while cruising around. 
 
I did upgrade the fuel pump wiring ages ago, directly from the battery with a separate relay and a 30A fuse. The fuel pump ran out of flow on E85 on the stock wiring before (it got to a point where increasing the injector duty cycle didn't richen the AFR).
 
Thanks for the tips about the surge tank. There are a number of reasons why I don't want to install one unless I have to. Fuel temperature, as you mentioned, is certainly one of them. Another is the fact that my boot is nice and empty at the moment and it would be good to keep it that way if I can.
 
That's a very good point about the possibility of too much restriction in the fuel lines and/or filter. EDIT: See my next post.

EDIT: I deleted my last two posts and combined them into this post with a few corrections and some important new details that I discovered.
 
I found this very useful chart while googling:

 
NOTE:
50 GPH = 190 LPH
60 GPH = 228 LPH
70 GPH = 266 LPH
80 GPH = 304 LPH
90 GPH = 342 LPH
100 GPH = 380 LPH
 
I had a quick look in my engine bay to confirm the following:
 
- The custom fuel lines from the fuel filter to the rail and from the rail to the regulator are both 5/16" (-5AN). I always assumed that they were 3/8" (-6AN) but I was wrong! The pressure drop between the pump and the rail (and then the regulator) might be higher than I thought, so the pressure at the fuel pump may actually be high enough to put it close to its flow limit.

- The hard lines above the fuel tank are stock. 
 
- Where the fuel tank hard lines end at the firewall, there's still an original fuel line connecting the fuel supply hard line to the original banjo fitting on the bottom of the stock fuel filter. This discussion has also reminded me that I'm due for a fuel filter change, so when I remove it I might cut it open for inspection, then depending on what I see I may consider a conversion to a stainless mesh fuel filter, which would be a good time to upgrade the fuel line connecting the supply hard line to the filter, and remove the associated banjo fitting which is probably quite restrictive.
 
- There are three 90 degree fittings in the fuel line between the rail and the regulator, and the fuel pressure sensor is on the regulator, so the pressure in the fuel rail may actually be a few psi higher than the sensed pressure at the regulator! Luckily it will be easy to remove at least two of the three 90 degree fittings.
 
- Given that all of the fuel lines in the engine bay are only 5/16" (-5AN), I think I should get them changed to at least 3/8" (-6AN). That should be plenty, but I could even go for 1/2" (-8AN) while I'm at it if there's enough space.
 
Since I've had no issues at all so far, any little improvement should be sufficient to safely allow a small increase in power.

Rob the pressure drop is not only related to the flow, its also related to the number and type of fittings etc. The smaller diameter results in greater fluid velocity, greater velocity results in greater pressure loss in the system. This will mean your pump will need to work a lot harder to deliver. meaning it will not last as long but it will also heat up excessively and heat the fuel i.e. the fuel is cooling the pump. I put in -8 in my system, right from pump to fuel rail

Thanks for the info Pat.
Yeah, I know that the fittings are just as important as the lines themselves.

If you are going to change them, you might as well go straight to -8 and never have to worry about them again. -6 is big enough for the return side

That's what I was thinking too.

If I do that, then the hard lines from the fuel tank will become the main restriction. Even if I don't change the hard lines, it would still help a lot having -8AN from the hard lines up to the fuel rail.
I'm still considering doing the hard lines too, but that would require heaps more work.

Sometimes dual -6AN is easier to package than -8AN, paricularily is you a feeding from a single pump in a tight area, and helpful when you have a twin feed rail.