H-pipe is pretty much lower end power and works best w/ chambered mufflers. X-pipe has more top end power (thats why i got mine, because when racing, your barely ever under 3k rpm)and usually works best w/ straight through mufflers (although i have flowmaster 40 series 2 chamber mufflers) Mac claims that there prochamber is the best of both worlds and some dyno reports show it makes more pwer than both. You can take a look here: http://fordmuscle.com/archives/2002/10/mac/index.shtml
I bet the power is neglible when racing and that other factors play a bigger role in it. On a dyno there are differences but there are also flaws in the dyno itself. The prochamber has been said to make the most power but i say to choose off of the sound you want.
George Klass at Accufab told me that they have done tests that show x-pipes do not make more power than h's. So who knows....
Aslo, the prochamber is not recommended for a lowered car, considering it will hit any curbs, speedbumps, etc. My cats on my x pipe on my stock height car even have some scrapes on them. It would be much worse w/ a lowered car, and even worse if i had the prochamber. The only reason i didnt go w/ the prochamber is because they didnt offer a version w/ cats when i got my exhaust setup.
Aslo, the prochamber is not recommended for a lowered car, considering it will hit any curbs, speedbumps, etc. My cats on my x pipe on my stock height car even have some scrapes on them. It would be much worse w/ a lowered car, and even worse if i had the prochamber. The only reason i didnt go w/ the prochamber is because they didnt offer a version w/ cats when i got my exhaust setup.
My car's lowered with Saleen springs and has a Prochamber... It's never rubbed or hit the ground, ever... That sounds more like user installation problems. If you're not fitting it up tight enough, I'm sure any mid pipe can hit the ground... Or if you're a jerk off and like to take speed bumps at 20 mph?