MPG-Mileage is a direct result of your right foot. The 5.0 and Eco will get close to the same MPG, with the 5.0 getting a tad better if you're heavier on the loud pedal, the V8 is more forgiving than the twin turbos.
Towing-The Eco will walk away from a 5.0 all day. I've towed with both, both are good trucks to tow with, but the max torque at under 2000 rpm make the Eco a beast. I've pulled both an enclosed 24' trailer and flat deck all steel open and the Eco does an awesome job.
Tuning- Ford can and will send out an Engineer or make the dealership send certain data if there is problem they deem to be a "known tuning related" issue. I've seen it first hand. If they can prove you tuned your truck, you're pumping mud. With that being said, I have plenty of customers that have tuned their trucks. As stated the throttle response and trans shifting is MUCH improved. I haven't seen too much gain in MPG from any of the tunes I've run across. Again MPG is a direct result of your right foot.
I've personally owned 2 Eco's. They run strong stock, tuned and will do more than you should in a 1/2 ton truck.