I forget but 2.5 inch dual is enough for like 600rwhp. My stuff is stock, and even if I got a cam and bolt ons, I would still be fine.
2.5 inch was also the limit in terms of size because I ran my exhaust up and over my trans mount, which even with 2.5 inch piping I had to shave a few bosses off the trans.
My application is custom though. So the only thing you can really take from my experience is that I am really happy with the sound of the mufflers.