I had dumped flowmasters on my stock H, and with my o/r H pipe. with the stock H it sounded like crap. it was very deep, and very muffled, almost like a honda with a fartcan. when I swapped in the o/r H it didn't even sound like the same car. it was still deep, but you could now hear the engine rather then just a muffled tone. I also felt a very noticeable loss in low-end, like below 1500rpms. eventually I got some custom piping done for my side-pipes, it sounds even better, and I got some low-end torque back. I like the sound of dumps, but not on a 4.6 mustang. leave it for the 5.0 guys.