It's all about correct exhaust velocity.
Just because you have 2.5" or 3.5" doesn't mean it's going to:
A) Choke the car out
or
B) Gains stupid amounts of power.
Backpressure is laughable and so is the debate about gaining power on a 275~ WHP LS4 car going from 2.5" to 3" is funny as well.