programmer question

[chevystudz7105]

does anyone here  use a programmer  on ther gas engine vehicles?  possibly anyone with a chevy truck with a 6.0l.  im lookin at buying one here soon but im not sure  which one is good  to go with.   i know a few people that run the diablo predator programmer  but they have diesel trucks.   and i know i wont get a big gain in power with them which is fine.  im lookin more for the ability to adjust my speedo cuz of my tire size  and also  how firm the transmission shifts  things li ke that.  but  any power gain or possibly mpg gain is always a plus.

thanks for any help