A tune just for mpgs isnt worth the cost. And you likely MAYBE see difference in 1-2 mpgs, if that.

If you can avg 28-30 mpgs all highway with a full tune up, thats about the best you are going to get. You may at times over 30-32 but the overall avg tends to drop slightly.

Also anything over 65-70 mph and the mpgs tend to drop, in a GT car you are already pushing 2100ish rpm and at 75+ you are just using more fuel with 2400-2500 rpm.