I'm going to toss in my vote for SLP, I love the sound of SLP LM1's with their catted X-pipe. At full throttle it sounds extremely nasty and you wont have to worry about the raspy/poppy nature b/c of the cats. Offroad X-pipes are a little too much noise, and are entirely too raspy for my taste. After listening to literally hundreds of Youtube exhaust videos for Mustangs I would also consider the Borla S type catback system, but you'll definitely have to spend a little more on that one.