I agree with most of your explanation for manifold pressure but I'm pretty sure you have still made my point. (I think )If the pressure above the blade has no bearing then at our given 65 mph level road load, the blade angle will still be the same as NA or effectively fixed. The denser air will in fact go around the blade but it won't be carrying any more fuel than it would NA because a carb meters fuel based on air volume not air density and the mix will be slightly leaner. Kind of like the difference between 2500' DA and say -500' DA with it jetted for 2500'. Would you agree that if you could cruise steady state at a -500 DA while jetted for a higher DA that there would be the potential for a slight increase in fuel efficiency?

And just to be clear, when I say I think a boosted engine can be more fuel efficient than the same engine NA I'm talking single digit % increase.

Kevin