Do you want to go fast ? Or do you want to climb ? Think of the prop as the transmission in your car. Do you want to be in 3rd gear or 4th ? Now, what type of car do you have ? Sleek and slippery, or blunt and fat ? The airframe generally determines the top speed of an airplane. An airframe will go just so fast. Above that speed, diminishing return gobbles up horsepower with little effect.
I have a Kyosho Cap 232. I tried a number of props. Same pitch, different brands and shapes. I tried different pitches and diameters. When I put the APC 11x5 on the plane (OS Max 46 ), it was an obvious leap in performance, climb performance. My girlfriend could see the difference. And you know what ? That plane didn't seem any slower in level flight either. I am a big fan of the APC 11x5 for 40 size aerobats.
Now, my Kougar ran a 10x8 (OS 50 ). Its a sleek airframe. It is capable of higher speeds. I wasn't interested in improvng climb performance. ( had plenty ) And there was a ground clearance issue with an 11 inch prop.
So there are generally other issues that come into play besides effeciency. An airplane is a group of compromises flying in tight formation.