I know this is an old thread but I removed my POS suitcase muff and plopped in an X pipe and here are my findings.
From the factory there is no crossover meaning no scavenging which means held back power and efficiency. The factory muffler is a giagantic, uneven heavy POS. It needed to go.
The sound is great. I have not gone WFO yet but I let it hit 4k and it's nice. It sounds more like a Challenger though. I gained a solid 1mpg and the engine seems a bit smoother. I've been driving home the same way for since I've owned this WK2 and I always put the jeep in cruise down this police ridden 40mph road. It is uphill slightly. It always drops into a lower gear to 2k. Since the X Pipe it does not drop down but stays around 1200 which means it picked up torque. I am extremely happy for the $160 I have invested.