MPG will start slowly coming up after break in. I started noticing the engine working better at around 3200 miles. Staying below 70 on the interstate should net close to 15.5 to 16.5 mpg if your careful. I don't know of any modifications that would improve mpg, really just driving technique. I tried a k&n filter, it did help with down low power but nothing substantial and mpg suffered also drivability suffered. It seemed to confuse the shift logic, I'd get hard shifts when I shouldn't of been. Put the stock filter back in everything went back to normal, not going to use anything but paper filters on this truck.
I got a 16.3 average over 400 miles on the interstate trips between Houston and Austin. Using cruise control, staying below 70 as much as possible, and using the brakes as little as possible all help. I basically have to re learn how to drive, you become more of an 18 wheeler driver than a pickup driver with the 6.0 if you want some efficiency.