Welcome!
Your Tahoe is nearly the same as my wife's (2005 4.8 2WD 3.42 gears). She has gotten as high as 25 mpg hwy, typically more like 22-23 mpg hwy. With the right configuration, these Tahoes can be pretty reasonable on gas for their size.
Iirc, 2004 doesn't have the e-fans or the chin spoiler, right?
I've heard good things about Blackbear tunes, but haven't spent the cash for one myself.
Black Bear Performance :: Custom Tuning Solutions for 96 and newer GM Vehicles I doubt you will see any difference due to plugs and wires over good OEM parts.
Good luck! And I'm curios to see what you come up with.