I've heard that a tonneau cover actually helps a bit.

 

That's true in a way. Though usually the extra weight of a hard cover will cancel out a little of the gains. A light weight performance style top would probably help. There's is one I seen on www.silveradoss.com. One of the moderators there has this cool looking cover. It's actually pretty flat looking then in slopes slightly upward at the rear where it meets the tailgate.
I thought it looked pretty sweet.
It's a ******* Speedsturr wing lid.
http://www.gaylordslids.com/speedsturr.php

Jim

 

Intake, LT headers/exhaust, e-fans, pulleys, and a good tune will either increase or at a minimum keep you milage the same. You'll drop a solid .5 to .7 seconds plus pick up some nice low end toque (assuming good tune).