go with 3" if you have some head work done which will be taking the increased amount air flow from the turbo and throwing more of it though the engine.

Port and polished heads
4 angle valve job
Lighter and 2 mm bigger valves
Bored exhaust ports to equal intake as much as possible

thats what i reccommend to have done to run with some more boost and successfully 3" piping. if you don't plan on doing any head and/or block work go with 2.5" piping.