I'm running a suspension maxx level kit on mine. It's not fully level, because I wanted to my angles in decent shape and i also didn't want to be nose up when towing/hauling. I've seen quite a few others say they run 305s without a level though.
I've probably dropped in mpg, but I'm all over the place in what i do with the truck...short trips, highway, towing, hauling, etc. so it's hard to really have a good feel on mileage. It's also only got 6800 miles on it, so it's not even fully broken in yet, so mileage will still improve a bit over time.