I'm not sure what's going on with that superduty. I've found in virtually everything that i've driven (plenty of 4 and 5 speeds) mileage stays up if you keep the RPM's down. Basically, if you flow more air, you burn more fuel. At a higher RPM, you're flowing a lot of air even at low boost. At low RPM's, you flow less air. I would argue that you flow less air at 1500 RPM's with 25lbs of boost than you do at 2500 with no boost at all.