Exhaust piping diameter

Did some searching on exhaust piping, but mostly found threads on which brands are better etc.

What I want to know is: For an NA gsr (usdm) with good airflow boltons, like a cai and 421 header, what are the general tradeoffs of piping diameter. Assuming same makeup, quality, mandrel bending etc.

For example, if a shop is making a pipe is there an advantage in going a litttle smaller like 2.25 inch? Or is 2.5 generally considered best? How about 2.75 inch (would this be good with a part time forced induction vehicle, like a nitrous burning daily driver) or is that too much flow for NA?
I am only really concerned with performance here, sound is not an issue.

I know that if you exhast is too big, you lose some back pressure, which inturn makes you lose a little bit of torque. but if you are running N2O i image you want a bigger pipe.

The general rule is n/a 2.5inch, bigger = you better have a turbo or nitrous. I've always been told 2.5 is a great size for n/a. I'm trying to find the ITR exahust (2.25 I think) for my GSR (usdm) because I want low sound.

Ok, so for a performance application, like SCCA, sounds like 2.5 is the way to go for NA. Thanks guys, just wanted to make sure. ^_^

NO!!!!!! 2.25 is good for N/A 1.8 litres and ait is the best of back pressure and maintain velocity of air travel for the integra and will help get power at WOT. 3.0 is good for nos and boost.

actually its 2.25 - 2.5 is good for na.