I'm interested in this as well. Been wanting a better tune for MPG for my wife's 09 Tahoe with the 5.3 as well. Blackbear is the tuner of choice I hear and after calling them I was a little disappointed to hear they could only promise 1 mpg improvement with their tune. Didn't think it was with the money myself but if there is other info I'm all ears!
You called? The guys on gmfullsize seem to say contact is email only. I've been wanting to tune my 8.1 for a while but never got around to it.