I saw no difference in mileage from exhaust. When I first got the truck, I installed DJ's economy tune and averaged 20.5 on my first trip running on the almost completely flat I-10 going 75 or so the whole time. After exhaust, I didn't see any real difference. I just went to Florida from north Kentucky and got 19.2 over the trip down with the bed full of crap and carrying 3 people. I got 15.7 on the way back with even more in the back and towing a VERY overloaded 12' Uhaul.
I've never seen 16 or 17 in town. More like 14. I usually average high 14s to low 15s mixed. I sure wish I had one of the trucks that did that well.