A very well designed 21/2 system should allow a full 500 hp with zero loss.
Well, alot of us gear heads are not designers so we stick to the std. 3 inch system.
I myself run 3 inch magnaflows and pipes or 4 inch side pipes off the headers with modified inserts that can still be removed.
This has been rehashed over and over the years and the three inch seems better.
That dufuss on road kill, not steve D, but the guy whose always doing the redneck builds, beating on new parts, ect has a spot on the hotrod channel with back to back 2.5 to 3 inch systems (kits) I think and the 3 inch produces more overall.
The OP wants to know at what point he needs to go up in size. It doesn't make sense to spend extra to put a 700 hp system on a motor that won't go above 475 or so. Referance points is all most of us offer, and he will have to decide if the future improvements can make use of it.
I put a 2 1/2 system behind a smallblock with a cross over and sonic turbos where the spare tire well used to be, and it is very quiet, yet will allow enough flow for 400 hp with very litle loss of power if any. Now if i decide to put my race car on the street, 3 inch may not be big enough.
