Engine Masters did a good comparison of mufflers and their effect on HP. I think it was this episode:
Anything straight through will have HP gains but will also be louder.
It's always a trade-off between HP and noise.
The cats won't hurt HP as long as they haven't been clogged by oil-burning but will muffle out the drone caused by a lot of setups. I don't think they actually muffle anything, but they change the resonance of the exhaust.