IMatt, I get about the same that you do with my 2007 GT. On occasion I will get 14 around town, but that's about the best it will do. I can get 26 on the road.
I had a 2005 V-6 and I can truthfully say it would not do any better than my GT on the road and about the same in town.
If it would do better in town, I would love it. Road mileage I cannot complain about. My wife's Focus gets 34-36 on the road, averages about 24. I have more fun in the Mustang.
All of these are realistic figures. I teach at a university and while the male students all love my car, it is hard to convince them to go green while driving a Mustang.