I have those on my car. With the long tubes and the stock exhaust from the cats back it sounds good. Not too loud at normal driving speeds and has a nice rasp at wot. Dont know if they helped witht the hp department cause there were no cats on the car when I got it. I added them. I hoped they would quiet the exhaust a bit when I put them on, but it did not really do much to lower the sound level.
You can also get them with either metal or ceramic substrates. I called Pypes and the tech guy said the metal would hold up better
on the road courses where I play.
I have had them on the car for two years so far and they are ok. I put them on behind the Long tubes where the exhaust pipe angles back towards the middle of the car and the H portion is. Welded them in, no big deal. GA
__________________ 05 GT Techco 7psi, 428hp/395tq, Tillman tune, 18x9.5 GT500 wheels, 14" Brembos, Steeda Front strut pads and front relo kit, BMR lowering springs, Tokicko D-Specs, GT500 Front lower control arms, BMR adj upper third link, BMR adj PHB, BMR rear relo brackets, boxed stock phb brace, Hooker super comp LT with Pypes HF cats, Stillen adj front & rear sway bars, Steeda sport shifter. | 
Mustang GT High-Flow Cat Question 
