The only SRT6 with an aftermarket exhaust that I know of is on NeedsWings car, and he built it himself.

You can probably go with a muffler from magnaflow/borla/flowmaster, I'm pretty sure nobody makes a cat-back system for the srt though. With only a few thousand srt's made, it's not economically viable for any of big exhaust companies to invest money and research / testing to come up with anything.