Yeah I've got my fingers crossed, it made a huge difference on another car with a similar problem.

I've got a new set of leads on there and its improved the symptoms but its still a bit sluggish under 2,500rpm, though not as bad.
 
The problem now is the coolant temp gauge is now going up and down and the engine seems to be responding as if the coolant is between different temps. I've replaced the thermostat and tested the temp sensors several times (and compared with spares), I can only think I have an air bubbles somewhere but I've bled the system several times and its pouring out the filling point. 
 
What else would cause the temp gauge to drop (gradually) under higher revs then increase when driving at 50-60 in forth?
 
EDIT: this definitely seems to be the cause of the problems now. Once the temp is sitting at its max (just under half, by about a needle width) it runs fine, seems to have good power and doesn't shudder as much. Once the engine is given a good full load rev in second to 60kph (so maybe 4000rpm at most) the temp drops strait away to about 1/2-1/3 the gauge and then it starts having problems again, shudders on launch (anything below 1500rpm) idle sits at about 1200rpm and power is lacking until under boost.
 
Really would love to know how the temp is dropping and how such a small drop is making such a big difference in performance?
 
Nick

Did you put an oem thermostat in it when you replaced the old one?
It is probably wide open under hard acceleration allowing colder coolant to flow into the engine.
 
When the temp drops the factory ecu is going back to cold start conditions. Running rich and less ig advance.

It's a Tridon highflow (same size opening as OEM) and was working within temp specs and opening and closing quickly. I'll be really annoyed if it's not working within spec in the car; Especially since I found 2 spare OEM thermostats after I installed it.
 
Obviously the in car gauge isn't giving me a very conclusive temperature assessment only an indication but though the difference is visibly noticeable I wouldn't have thought there'd have been that much of a temp change. I'm assuming we're talking 10-15degrees and this is enough to drastically change the mapping?
 
It certainly is running very rich by the smell of it.
 
The other thing I've noticed is that the car only has to sit without the engine running for a couple of minutes for the temp gauge to start dropping, it seems to cool down from running temp really quickly but takes ages to get much cooler. I should also mention that I'm just running water at the moment because of the short running periods, and dramas, to save me wasting my OEM coolent.

I don't know man, I've never had any luck with aftermarket thermostats. Seem to be more trouble than the saving you get from not buying OEM.
Is it a big job changing it in an AW11 4agze? I'd be trying the OEM one for a while. I think the problem will go away.

More annoying than anything else, takes about 45mins. I'll change it over tonight and see how it goes tomorrow. Hopefully this sorts it out, but I can't imagine anything else that would cause the temp to fluctuate, maybe the highflow was a bad idea.

Good luck then. Hope the OEM one solves your problem.

Partly resolved (hopefully), there was a still a fair bit of hesitation when I first pulled away, (quite aggressive between 2000-2500) then it smoothed out once it was up to temp. Temp seemed to hold pretty steady so I hope its all sorted. I tested all three (OEM, aftermarket OEM replacement and high flow) thermostats together and they all opened simultaneously, with the high flow was much wider, the OEM was actually fractionally slower to close but I used it anyway. Now I have given myself another problem: Gave it a quick squirt in second gear and my SCer tensioner pulley fell off. Now I can't find it and because it was custom made and I threw the receipt and box out I have no idea what part number it is. All I can remember is that its a cam belt tensioner pulley... great.
 
Thanks for all your help, I will update again once I've given it a good test. If I can get another pulley.

Sounds positive. If only the pulley stayed put long enough for a decent test :)
Hey no problem with the help, I don't actually own a 4agze but enjoy trying to diagnose problems for others.
Let us know how you go once the pulley is back on?

Can you drive the car without the sc belt? Or does it perform another function as well?

I heard the pulley come off so turned the engine off and pulled over but I couldn't find what was wrong (pulley is right down the bottom) so kept driving and heard the belt slipping so pulled up again and disconnected the SCer, and she ran fine for the drive home very sluggish compared to an NA 4A but power was smooth and revs felt good. I didn't realise the pulley was missing until I got it back in the garage, but unfortunately the belt runs the water pump as well so its off the road while I make another tensioner pulley.