My SD runs a Diablo Predator at near 400HP.
In the beginning, it was hard not to 'play some' and doubt that I ever saw any mileage gains then, but the power enhancement was very noticeable.
I've seen my best mileage gains driving smart like keeping max speeds down to 65 or so. Hard to do though on long hauls. The SD is like pushing 'box' through the air, and higher speeds tend to shave off miles. Also, idle times when I used to commute tended to eat available miles on a roll'n stop drive. I would try and skirt around traffic, and even when waiting for my morning 'joe' at the local coffee shack, I either don't let it idle for extended times in line, or kick it into neutral/park so not to drag off fuel with the converter at idle. Moderate warm up times, even correcting tire inflation pressures + (geeez I sound like Obama) since the factory fill is usually set lower to get a 'smooth' ride, but increases rolling resistance. Shop for the best fuel pricing helps too with the overall fuel pains. Sometimes I get lazy
and, I try and group my stops more 'these' days with the fuel costs rising to help with my monthly use and bill.
I doubt you would see a gain of 8 miles a gallon with any programmer or chip, think the best I ever saw was about 2 on a long haul (empty) if so, I'd be pretty sure the manufacturers would use a similar programming level (if possible and still maintain emissions standards) given the 'mileage wars' between models/manufactures.
Don't know how long if ever, it took to offset the cost of the programmer to rated fuel mileage savings, but when adding any power enhancement product 'does' come with some risk. My experience with at least the DP, was that I haven't lost any mileage with the power build under normal driving, some mileage increase depending on habits and condtions. When towing/hauling heavy, the program is retured to factory original so I can't add any claim there. Just MO..