i changed from the stock exhaust to a magnaflow highflow cat, 2" to the muffler, then 2.25" out and my gas mileage improved by about 2 mpg. i also got a lot more power. it seemed like i gained low end or midrange power the most( i can be going 35 mph w/ the truck in fourth barely touching the gas and it doesnt show any signs of struggling) which should be the opposite of what should have happened but im happy. my peak hp seems to be from 2,500-3000 rpms, which is about 35 mph in third gear then flooring it. i just need to get a connector pipe made for my lce headers so i can put them on since ive had them for about 8 months lol. its hard working full time and being a full time college student.

sorry if my experience doesn't help you much since i have a carbed motor and a 5 speed but i thought i'd give you a little feedback from my situation.