I used the search function and am dipping into some really old posts. I'm just curious if anyone has stock vs aftermarket catback dyno numbers. I realize that a cutout is best performance wise. I also don't fool myself into thinking any cat back is really gonna get you too many ponys at the rear wheel. For me, the sound is the main reason and if I gain 5-15 hp it's just a bonus. Am I wrong to think the best solution is pick the best sounding cat back and then just get a cutout for 400rwhp and below?

 

Cat back and the cutout are OK, but if you are looking for 400 HP you need a set of headers to go with them, plus a bunch of other stuff. The 5-15 HP increase is about average for an otherwise stock car, depending on which car you are dealing with. If the car is modified and needs more breathing room the HP increase will be grater.