I just ordered the pypes o/r x to go along with my loudmouth 1 catback. H pipe is more of a deep muscle sound, whereas X pipes are more raspy like loudmouths are compared to sumthing like flowmasters as a catback. Also, x pipes add a little more horses than h pipes because of the bends in the pipes.
This is a video of lm1 catback with pypes o/r x, thing is LOOOUUDDD!
[ame="http://www.youtube.com/watch?v=7zlYGfQmA-c"]YouTube - SLP LM1 Pypes O/R-X pipe[/ame]