One of the first things I did when I bought my MJ originally was to install a Mike Leach header, junked the dimple bend, ran 2.5" pipes all the way back from the muffler, and a K&N intake. It made a difference, not great, in performance and actually provided a bit better mileage. Unfortunately I had no before and after dyno readings for these mods. About a year later, the stroker went in, along with a 2001 intake manifold and Unichip. Along with the new stroker the same exhaust and K&N went back in. For all this I do have dyno readings here: http://www.jeepstrokers.com/forum/viewtopic.php?f=5&t=387 Pretty good results. My conclusion from all this is that exhaust mods by themselves really don't do much to justify the expense, especially on a N/A Renix, w/o other enhancements. When these other things are done, that's when the free-flow exhaust, along with better intake capacity and tuning to balance it all out makes it worthwhile. And I agree with Cruiser: there's no way a crush like that restricting exhaust flow on any engine can do any good. Except on two-smokers.