From what I've seen there is at least 10hp to be seen on the dyno with the x-pipe. Unfortunately I personally haven't dyno'd so I have no real data for you. Sorry!
Isn't the function of an "X" pipe the same as that of an "H" pipe, in that all they do is equalize the exhaust pulses from the individual cylinder banks?
And really, all that this "equalization" does, is prevent that annoying "blatting sound" at WOT and the chintzy gurgling sound at idle?
In the "old days" this was accomplished with a simple "crossover pipe" up by the tranny....but nowadays, space restrictions cause it to be placed well 'downstream' from that ideallic location. However, the effect is still pretty much the same....reduction in exhaust pulses, and not much else.
For what it's worth, I had the factory arrangement replaced with an "X" pipe and can't say I heard, or felt any kind of difference in the overall sound of the exhaust. I think it's more the "power of suggestion" taking over for those who claim to hear a difference between an "H" pipe and an "X" pipe.
No offence intended.