If you want more rumble/louder then you go catless. A high flow cat H or X won't make that much more rumble.
The X to H and sound is different between each car, what exhaust you have and so on. I've had H pipes on old school 5.0's, 4.6's and so on and they gave a nice rumble. X on certain cars and some gave a rasp and some sounded like an H pipe. Like the BBK X on my 03 Cobra...had rasp and hated it. Swapped for an Bassani X pipe and zero rasp and didn't sound like an import V8.
My car now had lethal X with ATAK axle backs but stock cats. Sounded like a nice V8 with rumble. Took the cats off and now it's louder but with a rasp at times or when its cold. Going with the 11-12 GT500 axle back to see what it sounds like. Though once I get my BMR K member, I have longtubes and shorty X to install too.
Power between the two it minimal. Usually 1-3HP. But from everything I read/saw and how my car felt after catless, makes it worth it. If you can. Are you going to blow every door off of another mustang or Camaro...no. but every little belt helps to nudge them out.
but to answer the question on high flow cats....resonator delete would make it louder, if you can't or don't want to go catless.
2014 Mustang GT, A6, Grabber Blue