well It was pulling slowly to 4000 then falling on its face with the factory 2" pipe/cat/muffler.

After updating I saw marginal improvments between idle and 2800 RPM but after that........ @2800-2900 RPM the exhaust clearly got louder and the truck pulled strong peaking at around 5000 but would pull to the redline with no problems.

In my case the exhaust was a real limiting factor with everything else I had done. I dont think you will see the kinda gains that I saw but I can say with out a doubt that the headers where a minimal gain with out the exhaust. It is actually more fun to drive when you can pull strong to 5k then shift and be right in the meat of the power again. With 32" tires and stock 4.10's I was able to run first second and third pretty hard creeping into the 60-70MPH range when I push the clutch in to hit 4th. It should be alot better when I get 4.88's installed

It seems to me that all the little things work hand in hand to add up.