[jetuomi]

okay, a guesstimate of mine is going to guess 7-12 HP..
the x-pipe gains over a h-pipe, but the cats reduce that and thensome..

(but don't hold me up to that, I'm just going along the fact that a 300 HP car with stock h-pipe that I knew of gained around the same going to an h-pipe... but you've got more HP, so, you might lose a bit more, to which the x-pipe adds a bit..)