Speaking of exhaust, I replaced my muffler recently with a Borla exhaust as I was curious to see what, if any, sound and fuel/power improvements I might see. Usually Borla exhausts are reserved for 8-cylinder engines to give a more "throaty" sound and add some fuel/power improvements and I have to say, my 4.0L inline 6 definitely sounds more like a V8 without the fuel expense of that engine platform.
As for fuel/power improvements, I doubt there are any and if there are, they are negligible. Anyways, just a short thought on this since I see the topic of exhaust has come up.