I remember modding cars years ago and telling myself and my friends that I put the new intake, exhaust, ____________, _____________, ___________, and ___________ on to increase my gas mileage.
I was kidding myself, my MPG went down with every modification; the power on the other hand was noticeable.
As much as it pains me to say this, Ford is probably spot-on with how they have these trucks tuned down to the diameter of the exhaust pipe. With the big push in the past five years for fuel economy, they really have to push the limits. My 5.4 got 20.1 mpg on a recent highway trip (including some mild 4x4in at the end of the drive). No way I would want to touch anything to make that worse.