The avg. mpg I get is not a guess and is hand calculated. I'm playing with my different tune options on the canned Superchips tunes and am keeping track of how the options perform. As for my suspicion of what you will get,14-16 mpg with a 4" lift, yes, that is a guess. Obviously there are a lot of variables involved. I wouldn't think there would be much difference in mpg from a 4 or 6 inch lift, I could be wrong. Forgot to add I have a 52" light bar and I did lose a little over .5 mpg. with that install. I suppose I have done just about everything to my truck to reduce gas mileage other than mounting an 8 foot brick wall to my front bumper, so maybe you will do better than I stated.There are a lot of guys on here running the setup you inquired about, so hopefully you will get more info.
Anyway, best to set the expectations low and if its better, then no complaints. I say ignore the mpg. and drive a truck you like. I wouldn't trade back any of my mods for a few more mpg. Between the costs of mods, the truck itself, regular maintenance and insurance, the few extra bucks on gas is pointless for me to create a thought war in my head about. These trucks aren't cheap. Lol...