The GT3540 is a very nice turbo, and quite a good price. The problem I have read about is they only come with a 1.06 exhaust housing which is way too big for an RB25DET (more suitable for a rotary and RB30 and XR6T (the XR6T is a 3540 but with a smaller comp cover, .50 instead of .70 like you buy from Garrett).
You can get the turbo modified to a .82 exhaust housing which is alot more friendly and should start to make decent boost around 3500rpm. Do not go for any of the .6x sized exhaust housings as these are too violent coming onto boost and you will just wheelspin everywhere.
A guy off these forums fitted one on his RB25DET by the name of NisJet, it made 210rwkw on 12psi. His car has 3 inch exhaust, intercooler, power fc, fuel pump.
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Do a search for GT3540 and you will see a few people that have run them. Most that keep the large 1.06 housing have forged internals and need to rev the car to 8000rpm to make good power out of it. Also does that price include a new manifold? I think it was Roy on these forums a while ago wrote you need spacer plates to put it on the stock manifold or else the comp cover will hit, and it generates heat spots and warps the stock manifold.