I think the loss in mpg will be more likely caused by a heavier right foot.
I average 22-23 combined before the VTC...
Right now, with gas as cheap as it is its not an issue with me.
I'll know the mileage difference in a month or two when I make another road trip..
Right now, i'm enjoying a 3500lb car with a V6 that will spin the rear tires without breaking a sweat