Outside of forced induction, the best gain I have ever seen is long tube headers. They are spendy but really help in the tq area. I have read that custom tuned chips that take some of the fail safes out of the tranny tuning can do wonders but I have no experience with them.
On my 03 Yukon, long tubes made a very noticeable difference. I didn't dyno the truck before hand and I didn't do it after so I have no number to prove it. But the truck runs better, cruises at a slightly lower rpm and seems to down shift less.
The only way your truck can cruise at a lower rpm is if you're driving slower, or if you've changed the gearing.
