I have a 4-door Chevy Tahoe 4x4 with a 6" lift, 35x12.50's, a hypertech programmer, and a K&N filter.  My engine is pretty much stock (350 V-8).  I would like to be able to get more power and a little better gas mileage out of it.

Anybody got any good advice like maybe different gears, dual exhaust, maybe something else???

I assume you have either 3.43 or 3.73 gears in it so gears wont help. best bet is putting headers on it and running true dual exhuast which will end up prolly being 2 1/4 at flanges from headers to 2 cats to 2 mufflers with a 2" or so H in there somewhere..  so that you will now have 4 1/2" exhuast over chevys weak 3 1/2 or so to cats then to 1 muffler other than that you got a 6" lift with huge tires. You dont get gas milage