the gains you would see from a muffler would be minimal at best. more air out = more air in so if theres a drop in backpressure you should see a small gain, but mostly it would be placebo effect. ubend delete and resonator delete have seen dyno results of up to 7hp. im running stock exhaust with those two mods and a magnaflow cat. i love the sound, and the performance certainly didnt decrease.