if you dont have any other mods like LT headers, Cam or FI, there is really no HP differences with an OR X or H. Maybe 1-2 HP.
Any OR mid pipe will sound different depending on your axle backs. X will be more raspy than H. However paired with a good axleback, you might find that sweet sound you are looking for.
Generally speaking, a chambered axleback will sound better with an H , and a straght through axleback will wound meaner with an X , however there are some exceptions.
2008 Metallic Alloy GT Convertible Premium
Show : black top| 18" bullitt polished wheels| CDC light bar and black billet replacement grilles | black/amber corner lights | GT500 rear spoiler| AH chin spoiler
Go: FRPP CAI with StangHi tunes | Kooks LT's | FR500 axle back | Steeda Sport springs | Koni STR.t struts/shocks | J&M camber plates | UMI adj. panhard bar | LCA relocation brackets | GT500 LCA's | Hurst Billet shifter | 3.73 gear | FRPP axle girdle | Baer Rotors and Pads