It's not really a well-known exhaust system, so there probably aren't many people running it. I can tell you from looking at their site it appears to be a knock-off Magnaflow muffler (a smaller one) and the site lists the sound as moderate/aggressive so it will probably be fairly loud.
XJs and TJs don't have much for sound deadening inside, and they also have very simple exhaust systems (no additional resonators like newer cars).
How is the condition of the rest of your exhaust? I can say the most mellow sounding exhaust I had on my XJ was a 18" long Magnaflow oval muffler and a stock 2.25" system. It had a rumble but wasn't annoying.
For an entire exhaust replacement, Borla is known to be fairly quiet with a low rumble, but they can also get expensive. Magnaflow or Borla systems are probably the most reserved, and anything else will likely be quite a bit louder. I looked on Quadratec and they have cat-back systems from OMIX that resemble the stock factory system. I will probably go with that once my TJ exhaust rots off. Even at the young age of 30, I'm done with loud annoying exhausts like I had on my XJs.