I didn't see a mileage hit really. However, getting them balanced and keeping them balanced has proven to be a real pain. I think I'll be looking elsewhere next time.
I would hope that the extra 6 lbs per tire doesn't have cause a significant drop in MPG. I've looked at some other lighter tires like the Toyo OC ATII but they seem to be similar to the stock Dunlop AT20 that I already have.