For the 2.5L, 2.25 = 2.5" for most purposes.
I have looked at the 2.25 Bassani gains side by side with 2.5" SCA and Brullen...gains same across the rev-range within 1-2 HP. Average gains above 4K RPM same (about 6 HP) Peak gains same...SCA 6HP, Bassani 7HP, Brullen 10 HP...however, this is misleading because in the pre-Brullen comparo they did not go to redline and it was still gaining power. At the last point on the curve where both plots were present Brullen was at +7 HP.
These were all on a 2.5L SVT with intake mods but not headers. On a 3L or FI, I think 2.5" would be clearly better, MAYBE on a heavily modded 2.5 (but I doubt by much). Agree with Demon that if the stock SVT 2" resonator (the big prob) and mufflers were replaced with Magnaflows it would prob be comprable (as the stock mandrel piping and Y are fine) but I do not think this has been dyno documented.
