I am running Matt's custom tunes with an SCT tuner. When I'm not towing my trailer and using street tune, AND I keep my foot out of it, I can get a couple MPGs better than stock... From 16 to about 18. If I'm in any kind of a hurry and my right foot gets heavy, that couple MPGs quickly go away and I lose additional MPGs. I probably don't want to know what my mileage is when I step on the go peddle excessively in street tune.
When towing my 12K trailer and in "unlimited tow" mode I get about the same as stock. Right around 11 MPG.