I can vouch for the diesel excursion (I have a 2000 7.3L) that I bought in December. I have the Superchips Microcontroller loaded at +70HP (320 total) and recently checked 19.1 MPG on a highway trip (not towing). I used it to tow both of my cars to Nashville in a 32 foot TAG at about 12 MPG:
It is amazing how much power and torque you can get out of one of these things.