3 - 3.5 inch exhaust is excessive, even for a turbo car. I would recommend 2.5 - 2.75 inch exhaust for a street car.
for a turbo 4 litre engine? i dont think so!! 3" minimum for a XR6T!
thchoo - where do you live? if its near brisbane you could try redcliffe dyno - they have a XR6T with stock turbo and 15psi boost making 447hp at the wheels...
unichip do a chip for the XR6T, gives it about 2 more psi through the rev range, and i think that equates to about another 40kW at the engine.