I'm not sure about mileage but probably ~10 hp difference. However, the biggest difference you didn't mention between the two. The biggest difference to me is the difference in driveability and the fact that the truck shifts correctly.

 

not sure about trucks but when i used the canned tunes on my 2006 mustang gt they gave me a bad knock and shake for about a week until the PCM relearned. bought a mailorder tune after that and had no problems. then i decided to have it dyno tested and custom tune files made - this is DEFINATELY the way to go.