Some magazine did a full exhaust upgrade to an 03 Mach, and they gained 23 rwhp. That was with long tubes, o/r mid-pipe (cant remember whether H or X) and a catback. If you want sound clips, go to mustangexhaust.com, go to the forums, register, and then you should see V6, 5.0, and 4.6L clips. They can help you decide. A rule of thumb when selecting mid-pipes is that an x-pipe is more raspy, and an h-pipe is deeper and sounds more retro muscle car. Lemme know if you need more info.