I wouldn't rush into removing it right off for a couple of reasons.
It's no real restriction when open, given the cross sectional area of the combined manifold runners, and any slight turbulance it may cause pre intake port, is actually benefitial to the combustion process.
The second thing is, on a basically stock engine, when working correctly, it will increase max torque marginally, but it will bring peak torque on a full 1000rpm sooner.
As mentioned, up to around 225Kw it's well worth keeping. I'd expect when using a laggy turbo such as a hi-flowed ct26 or ct27 there would be also appreciable gains to be had keeping it.
If it's working ok, and holds closed when you apply a vacuum source to it, you can monitor its operation by fitting a LED to the dash.

artymr2
On mine, I could get it to open by driving in first gear at say 2500rpm at a constant speed for 5 seconds or so, then full throttle.  It wouldnt open always at the same revs, but did open everytime I did this.  If I just used full throttle while rolling off the clutch it wouldnt open. 

This has been my experience exactly, and I always believed it was either the butterfly shaft or solenoid sticking.  Having removed and inspected the T-VIS plate and given the solenoid a good workout on the bench, I found that it was all working perfectly, the vac tank had even held a good vacuum for days after it was removed!  Using an RPM switch as I have to override the ECU above a set RPM (I've set 3800 as a start point) will eliminate this seemingly counter-productive behaviour of the ECU, and as MuMan detailed above will allow the T-VIS to provide the performance gains it's capable of.
 
I guess like the apparent limiting of boost in low gears that Celica owners report, this T-VIS behaviour may have been designed to limit drive train torque in an effort to reduce warranty costs.  History seems to have shown us though that this isn't really necessary!

Good points guys, I'm going to keep it functional (override it with the ME3) after reading this too:
http://websworld.org/marcel/tech/tvis/tvis.html
What I don't understand is that if TVIS was so good, why didn't they use it on the Gen3 3SGTE?? cost cutting I suspect.
 
The GT-FOUR?? haha, Toyota really knew how to put a dampener on the party, its the lower gears where you want the boost especially in a 4WD...no wonder an Evo from the same era would have slaughtered it. 
 
 

guys
just one question, if going standalone what difference does it make if the TVIS is disabled. couldn't the bottom end torque that's lost always be made up by tuning the aftermarket ecu appropriately or even by using an appropriate boost controller to bring on boost quicker avoiding waste gate creep thereby kinda equating it to the same as having a tvis
what's your take on this. im also thinking about taking the TVIS off for ease of tuning on a PFC

Yes, I would say you could get a bit "back" by doing the things you mention, but you cant beat the laws of physics!!  IE If you keep it, you should get even more by doing what you are suggesting.
 
With the PFC, if you hev no afm, you will need to pick a rpm switch point, and tune both sides of it, due to nto being able to cater for different maps as the VE changes, as mentioned by cogs previously.
 
 

/\ yeah thats what im trying to avoid, tuning for both sides of an rpm switch point. then comes getting the switch and wiring for it to work etc etc.
if i can get my turbo to spool quicker than stock then iil be happy to get rid of the tvis which should be possible with a decent boost controller.

Yep a decent EBC should be able to hold the wastegate shut to eliminate wastegate creep. This is called Internal Fast Spool on the ME3 and I'm looking forward to it.

 

I doubt you guys are going to have much success improving turbo spool if you disable the T-vis.
As cogs explained, the T-vis effectively increases manifold runner velocity, which in turn improves VE, or more accurately cylinder filling effiency and this in turn improves turbo spool.
Should you disable the T-vis, an efficiently sized turbo will take longer to spool regardless of optimised tuning, as precise tuning of ign & EFI values will improve power by increasing combustion efficiency &  raising PCP closer to TDC, but won't effect VE.
Better to leave it operational and enjoy the increased torque and quicker spool it provides when working correctly, stock ECU or standalone.
 
Should you decide to take manual control of the T-vis as I've done (no choice on a gen3 -:)) you can set an RPM switch point, using an rpm switch that has hysteresis (or ECU2/3), and include a simple boost over-ride circuit, or pressure switch, to open at a selected point according to the  spool characterists of the turbo your using. This way the operation of the T-vis becomes predictable and more beneficial, closing off idle and opening prior to VE peaking.
 
I tuned this arangement basically with the T-vis locked closed up to the switch point, and locked open after, with some enrichment at the switch point itself. This worked out very well, and delivers good power. Recently using 20% bio-flex, it worked out exceptionally well, so might be a good starting point if you want to experiment. Down side is your removing the T-vis operation as an effective safety strategy, but then I'm not convinced its all that effective as such, and more likely used simply because it's there.
Also don't forget USDM ECU's and JDM's are different, the nominal switching point for a USDM is 4400rpm, where as JDM is 3800.
 

Hey cogs, did you use the IGT or IGF for the igniter?

IGT is the signal you intercept for timing control.  IGF is just a feedback signal that the ECU monitors to determine whether the igniter and coil are functioning correctly.