Interesting.
I have done a number of reasonably careful tests and cannot see any impact of varying revs 2200-2500 on the MPG, if I set the fuel flow to be a constant figure.
One would expect the reduced friction at lower rpm to surface somewhere, but it doesn't seem to.
If there is a difference it is less than 1-2 kt.
Maybe I didn't re-lean for peak EGT carefully enough at each new rpm setting. Must try this again sometime.
Piston efficiency varies in a strange way with rpm. Some, apparently, are more efficient at a higher rpm.