you wont be beating anything witha 3.5 inch exhaust haha ur turbo will be spooling al day with no back pressure to be able to push the car mate haha 2.5inch turbo back resonated and ull see good results 
pray tell how a larger pipe AFTER the turbo will affect it's spool.. when the manifold, and therefore "back pressure" before the turbo.. where the spooling force comes from, is left unchanged

I don't mean to sound like a miserable c*nt but this whole back pressure myth (created by honda owners i believe) really bugs me... exhaust pulsing and gas velocity really isn't that hard to get your head around if you know the basic workings of a 4 stroke motor.