Shooting a single shot in full auto isn't more efficient. Shooting in full auto is. The startup surge of power for a single shot whether the gun is set to semi or full is the same. With semi. When firing a sustained full auto burst, the motor is already wound up and doesn't have that startup surge with every BB fired. That only happens on the first shot.
So whether in semi or full, squeezing off a single round has the same effect. It's not determined by what position your fire selector is in or how much to press a 2 stage trigger.