The standard engine of a gtt sucks...And I can know it. The engine is very strong in terms of power and pressure ( the engine block was build for 250 hp) But the rest of the parts is not made for tune ups. The standard turbo (t2) can run 1.2 bar max. That is 16 psi I believe. The problems with cooling is known I assume.
About the Intercoolers...try to get in what fits. But the enginemanagement is not compensated for cooler air. So at cruising speeds you will loose a lot of effiency.
Keep the engine standard and drive it like a turbo need to be driven (warming up And more important cooling down) Do the maintance often (every 5000 miles changing oil, filter) And you can have a lot of fun.
Do you want more go EFI with programmable ignition. Like BBperformance does.
Grtz,
Derby