It is true though! Too much prop is simply not efficient. My understanding of this is that the injection pump injects the correct volume of fuel for the engine to obtain a certain rpm. If you are swinging too much propeller (or loading the engine too much) it never achieves the rpm and thus the efficiency and power are lost...
I have never calculated my seasonal fuel used vs mileage sailed... We do sail a lot so I'm sure the numbers begin to look better...