That my ebay auction - I'm also a sponsor vendor here. I have placed several of the JK accessories on Ebay in addition to the website in order to reach a broader audience.
At least one customers has conducted very specific before/after test using a dyno. There was a marked improvement in acceleration measurements. Like on most cars, a free-flow exhaust can be further "exploited" with the addition of a cold-air intake.
In terms of clearance, the muffler sits just slightly above the frame rails in a very protected position. The primary objective in the design of the kit was to improve off-road capabilities - substantially by removing the muffler from its stock rear location.
The muffler is 100% stainless and has a lifetime gurantee.
You can find a few exhaust clips at this location: http://pages.sbcglobal.net/kpond/CoolTechJK/Exhaust/
Please note that the exhaust sounds louder in these clips than it does in person. For example, compare how loud the starter sounds in the start-up clip. It's hard to calibrate your ear for this.... but perhaps the main thing is to see if you like the tone.