On a naturally aspirated engine, I would agree that a single 2.5" pipe can work at 400 HP, but with a turbo, reducing back pressure makes power so I'll be looking for a 3" system or making my own.
A mild, mostly stock car would do fine with 2.5" but upgrade the turbo and suddenly 3" seems like a good idea.
A good stainless system can be made with thin wall tube which minimizes any weight penalty.
The real issue is sound, bigger pipes generally equal more sound so performance vs. unwanted attention is how I'm going to balance things.
A mild, mostly stock car would do fine with 2.5" but upgrade the turbo and suddenly 3" seems like a good idea.
A good stainless system can be made with thin wall tube which minimizes any weight penalty.
The real issue is sound, bigger pipes generally equal more sound so performance vs. unwanted attention is how I'm going to balance things.
