10 mpg towing is about right. I got 13 mpg at 65 towing a grand cherokee on a big ass gooseneck once, but that was fenton to detroit, flat land....
I've said it before, the big advantage to a diesel is the unloaded mileage over a similar 3/4 ton gas truck. Both get the same towing. Both have the power to tow, unless you are going super heavy then the diesel does get the advantage but for 1-2 "normal" rigs a gasser 3/4 ton (think 6.0 chevy, or v-10 ford type gassers) tows just as well.
When you get into towing consistently over 10,000 lbs the mileage is about the same but the added power makes the experience better with the diesel. (I hate listening to trucks downshift constantly)
Same with comparing a half ton truck to the superduty. The 7.3 will just yank whatever all day, the 5.3 will make a lot of fuss....but the 7.3 won't get much better mileage.
I highly doubt you'll see 16 mpg towing with a tuner on the 7.3, i've rarely seen those kind of claims actually backed up. Even if they are it's typically a best case scenario of a tailwind, a huge downhill, drafting a semi, with an empty 1 place snowmobile trailer.....