I've got the 5.7 with a 3" lift and 34" tires. Which creates about a 5% difference in mileage. Using the computer I get 13.5 around the city (14.1 corrected) and on a hunting trip this past weekend the computer showed 14.8 (15.5 corrected).
For as big of a truck as it is and the power this motor has I think the gas mileage is awesome.
I've looked over at the f150 forum when the ecoboost came out because I was interested. They aren't getting any better fuel mileage. In some instances it's worse. But that's probably more to do with driver than truck.