Without existing bolt-ons or gears, HP increases may be minimal but that is not what anyone is focused on at all. The drive ability is improved so much that it in turn effects acceleration, mpg, and all overall performance. The HP numbers are not as important once you get behind the wheel and the car revs impressively and seems to move twice as fast as it did before. Its also a necessary because it puts everything Ford threw into the car, placed at its maximum and most proficient places. You just have to have one. Its incredible really. A dyno tune is most efficient with cars with some mods. American Muscle makes sense on a nearly stock car because they'll give you re tunes for free every time you add some kind of upgrade.
Hope that helps- Have fun with the tuner, it does so much more than I even mentioned!