As for Single Blade setups most of the CL speed guys at the 89 Nats were running singles.
And now they don't, because the drag of the counterweight was dead drag with no thrust. The loss in efficiency is very small, hardly noticable, in full scale you would have to do carefull measurement of distance and fuel to notice the differance. What we are after is performance and often a muli blade prop with lower pitch and the same diameter will give much better thrust, or a smaller diameter with the same pitch will give a bit more thurst at the same effective speed.
The real problem is selection, to have a multiblade prop that will replace all of the two blade prop's you would have to have the same if not more sizes and pitch's. Instead there are less. Invaribly someone does the rule of thumb selection of 1" less pitch, or 1" less diameter, and it is not really the right size for that plane and engine. They assume that multi blade propes are worse because of less efficency and broadcast that over the net.