the 'stang forum consensus is something like mid 30's for the ecobeast, high 20's for the v6, and high teens for the manly version.
gears + blower it'll be single digit mpg easily.
I used to get better with my supercharged 11 than stock. Of course I didn't use all it had every time I left a stoplight either. When you have 600 whp on tap, discretion is a necessity. Lest you end up in the ditch quite often. Or on someone's bumper. Or in their back seat!