I suppose methanol and nitro are cheap so efficiency may not be important to some. Posts above indicate that a modeler might spend a good amount of money for improved fuel efficiency of a four stroke engine, why not take it to the prop? Propulsive efficiency is power in the fuel/power output. That includes prop and engine. So if you spend hundreds extra on an engine, what is a few bucks trying a handful of props to get the performance you desire? Optimized you would pay for the props pretty quickly in reduced fuel comsumption.

Prop efficiency was very important in electric model prior to LiPo. That's where I'm coming from.

I wouldn't consider a Cessna to be a draggy model aircraft. I was thinking in the model realm of a WWI biplane or triplane compared to virtually any sport model. You raely see a prop with a P/D of greater than 50% on a WWI model.