I've been digging Google, and the forums for this info. Been trying to find it for a couple of days. I'm looking for actual power gains to go along with the APN manifold. A chart, or test.

I'll be using a Thunderbolt spun cat (highflow) from preformance curve. Also a Walker SoundFX muffler, simply because I want to keep it mild. Future upgrades include; 703 injectors, and K&N drop in filter. The last piece of the puzzle is the exhaust manifold. Should I keep it OEM, or go APN?

I'll also be throwing on some motor mounts, hopefully future cracking wont be an issue.

 

This is really the only thing I've found helpful.

Header/Manifold Crack Issues
Myth: All 4.0Ls will crack the factory tubular exhaust manifold at the collector. The extra air getting to the O2 sensor will make the Jeep run rich, foul cat, and run like crap. The fix is to add an aftermarket header for more power, durability, and longevity.
Our Thoughts: We agree with the above. The exhaust system of a 4.0L Jeep has to deal with very high temperatures due to the 4.0L's lean air/fuel calibration that, when coupled with a long and inflexible intermediate tube that leverages the tubular header, is a perfect recipe for cracking. In fact, even some aftermarket steel and stainless steel headers may not prove immune.
Claimed Gains: 5-10hp and long-life
Actual Gains: 5-10hp and long-life
Notes: Despite its fairly efficient design, you will notice a seat-of-the-pants improvement with an aftermarket header when used in conjunction with an after-cat exhaust and a cold air intake. Just buy the header with the thickest gauge tubing you can.

Read more: http://www.jpmagazine.com/techarticl...#ixzz20HA6oRH6