Well you will prob. get some various up and down responses... Alot of people will say you can't really improve on the 4.0L without doing something major like a supercharger or stroker... While others will give testimonials about getting some performance gains for bolt-ons...
Me personally, I've done just about every one... intake, bored throttle body, headers, high-flow cat, and exhaust. Only thing i've never tried is a computer chip or module. I can honestly say my jeep is faster than a stock one, but to me the cost was not worth the gains...
There is a great writeup on that kit on rockcrawler.com, with Dyno's showing 21 HP gains (doesn't say what rpm thats at) Here is a bit of it:
"Before we started work on the Wrangler Sport, we took to the road for a seat-of-the-pants drive and some time tests. We also climbed a couple of hills outside of town, just to get a “stock” feeling. From 0 to 60 our average had been 12.6 seconds, and from 50 to 70 took 11.6 seconds.
With the Trail Jammer kit installed, our 0 to 60 time dropped to 10.9, and our original 50 to 70 time of 11.6 seconds dropped impressively to 9.9.
The real difference in feel was on the highway and climbing. The engine seemed more responsive and alive. On the dyno, we saw our horsepower jump from the stock 156 to 177 - a substantial 21HP gain for such a simple installation. The original torque climbed to 232 ft-lbs"