What sports car is not either gonna take premium or get not great gas mileage or both?
Get a geo or a base civic if gas mileage is your number one concern. I didn't see the ms3 mpg but I bet it'll be AT LEAST 20/27 which is plenty good.
Usually when you buy a sports car, performance is your number one concern (while keeping price in mind of course). Why would you even want a sports car if you're gonna shift by 2800 every time to have good gas mileage?