Ok, 3" is fine for a down pipe upto 550+ bhp if well design'd. but that dont mean you wont get more BHP if a 3.5" is fitted.
And a 3" zorst after down pipe is good for at least 650 bhp is well designed again.


2.5" down pipe is fine for 350 bhp, and 2.5" exhaust is good for more,

I've seen a pulser run 400bhp on 2.5" mongoose, on good boost and didn't seem to be a problem, Also saw a mate's pulser get 296 bhp on a 2" system.