Are multiple rotor blades more or less efficient than a single large rotor?
Also, could a normal helicopter be adapted to use these control systems to make them easier to fly?
Yeah they are less efficient. The reason to use them is that they can be fixed pitch and you get control by controlling their speed, which is very simple to do (it's basically a huge quadcopter). Normal helicopters keep the speed roughly fixed and get control using variable pitch, which is obviously far more complicated mechanically.
Don't know for sure, but considering you can run the blades at higher RPM due to the smaller radius vs approaching the speed of sound, it might be better. (also redundancy).