Well, the other factor in the mileage is how you use it. At a given speed, the power required does not change. The difference there is how the jetting and things like power valve threshold and the mileage can go either way with 1BBL vs 2BBL for a given cruise setting. Put your foot in it all the time, and of course the mileage will go down with more breathing capacity.
Again, drawing broad conclusions from one user experience is not valid.
