Wait4Me Performance
I have a 04 GMC 2500 6.0L auto. I have used Hypertech and Superchips with limited improvement most of that from the shift characteristics, so I would not recommend those. The one handheld tuner I have been impressed with has been the Bullydog gas tuner. I bought one of the first gen versions for my wife's 01 Suburban and it made a noticable difference in both torque and HP with improved MPG if you drive it right.
After I got the 04 GMC I started looking into custom tunes and found Wait4Me, for $160 he sends you a factory ECM fully custom tuned per your requirements just let him know what you are looking for. I had him remove all the torque management, abuse management, raise the shift points and firmer shifts, adjust for 35" tires and tune for improved TQ and HP on 89 octane. It also improved mileage by 3 mpg if you keep your foot out of it. It takes about 10-15min to swap computers and you put your old one on the shelf. He also offers lifetime free updates if you want something changed just send it back and cover return shipping. I've been happy with the 89 octane tune but may have him try a 91 oct full performance tune.
