I have the S1025 package. The mileage has gone from high 12s to high 8s in a mix of city/highway driving. Trust me, it's going to hurt the wallet if you are going to do long trips.
I don't hammer it all the time nor do I drive it like a Prius to gain the best mileage. I am on E85 but tried 93 and did not get much better mileage.
I am in Naples FL, it's hot here and been taking it to the dragstrip twice a month. It's a blast but I would not be concerned with heat issues.
As others have said, I went from 3.1 to 2.6 0-60 and low 10s now in quarter mile (my license plate is LOW10S lol). It's an absolute blast but I would not have done it if I was going to do more than 500 miles per month with it.