I had an o/r h on and my 06 GT with the flowmaster axle back and it sounded like crap. The drone was horrable, it was hella loud, high pitched, and raspy beyond belief. It was to the point of being embarrassing. I drove the car for 2 months like this and bought a o/r prochamber and man what a difference. No rasp, quieter that the h pipe by a good deal, and a very deep tone. There is still drone but it is much reduced. I have never heard one with a o/r x but if it is more raspy than the o/r h-pipe I have pitty on the person that buys it. I have never heard one with high flow cats either.
Due to the muffler placement on the 05+ you cannot compare the sound of a exhaust combo on a pre 05 car to what it will sound like on a 05+. What sounds great on a older car may sound like crap on yours.