Installed a Thorley HDX-635 this past weekend with a Magnaflow 50899 front catalytic converter y-pipe and Flowmaster universal rear cat. It's attached to a MBRP XP Series T-409 cat-back exhaust, a 63mm throttle body and an AEM cold air intake. (Jeep is 6-spd with 33s & 3.73s)
The intake, throttle body and cat-back exhaust were already on my LJ.
The headers and y-pipe rock!!! They're expensive and a pain to install, but I would do it over and over again. The Jeep MOVES now. Can't wait to try it on the trail, but it's a stop light dragster now! The headers also sound great; there's a throatey warble that wasn't there before.
Definitely worth the price if you're into power from a 4.0L!!!