things were quiet around here and then all of a sudden there's 6 threads on extractors!

a search & read would have answered your question, but I'll do it too.

I've got a TJ AWD Sports - replaced the muffler when a hole formed. Lukey 2779 or something and it's good - a little more noise than the open Sports/VRX muffler, no change in power. If you have a restricted muffler you should get a gain in power.
I've just installed HM headers - recommended - I did it myself, bolt in job, saves over 6kg of weight.
I didn't notice a power increase, from just seat of the pants driving, but will one day go to WSID and compare my 400m time when all I had was the muffler.

Overall, my AWD is not slow, overtaking is fun & easy, mind you I do ring the tits off the valves & cams so either you're being soft on the accelerator & revs or you have some kWs to unleash with a good exhaust.

Cheers!
Graham.