Horse shit, in a R32 GTR the dump pipe - front pipe and cat is the biggest gain even on stock turbs, it really wakens the car up
Build your exhaust for any future possible mods, there is no such thing as to big when it comes to an exhaust, just what you can fit and how loud you can handle it
My recommendation is dump pipes with 3" outlets, twin 3" front pipes with 3.5 outlet and full 3.5 exhaust unless you plan on going 3ltr then go 4" instead of 3.5