I wouldn't call it efficient just yet. High boost engines under high load (like most Raptor owners MO) will consume far more fuel then a larger NA engine. The smaller engine has to work harder and use more boost in order to generate the same poke as the larger 'less efficient' engine.
The key to efficiency is putting the largest possible motor in the smallest possible car, hit the nail with a sledgehammer. We've ended up with an odd trend of the smallest possible engine in the largest possible car....
Funny, we could potentially save on the initial cost and running costs going for the older 8 if it overall seems like a better package for cost.
Extra money left over for goodies.
That's only if you're driving FLAT OUT everywhere. And considering most miles will be on the street, nobody needs to be driving FLAT OUT in a 2016 Raptor everyday around town. The occasional blast won't kill the overall fuel economy of the truck.