This has been the subject of several conversations and the consensus is that no one has gained more than a few HP. So go for the sound you want, and be happy. I am in the process of modifying my exhaust by deleting the back CATs putting in a Y pipe, then into a 3 in by 12 in resonator then 3 in into the muffler, bypassing the resonator. I think that the dual out system mods may actually flow too much, lowering the exhaust velocity, thus lowering the scavaging effect. By going from dual 2 1/4 to a single 3 inch may get the air velocity to a level that will aide in clearing the exhaust and maybe gaining a few HP.