It is if you are running it hard and STILL getting that kind of mileage.
For example, my mothers 5.4L 2001 F150 4x4. When she was driving it, she averaged 310 per tank, babying the heck out of it, untuned.
Now, since my mom injured her right knee, and cant drive quite yet, my stepdad has been driving it. I also tuned it for her right before he took over driving it. He averages 300 miles per tank, and he jackrabbit starts, hard passes, and heavy brakes at EVERY light, never doing the speed limit. He complained to me one night about the mileage not improving since I tuned it.
They keep a mileage log in the truck, for business purposes. I went back and showed him the average with her babying it, and totalled the average up since he has been driving it.
With his hard driving habits, and still getting that kind of mileage out of it, he has been happy since me proving my point.
Typically, your mileage will drop when you first tune the vehicle, as you are getting used to the new throttle response, better shifting and more power. Once the "new" wears off, your mileage will increase over your previous average.
