I agree that for the RC 1/10 application, using a shifting gearbox is probably not going to be any benefit to the overall performace of the car.
If one could be produced, that was extremely effecient and lightweight, I would suggest a constantly variable transmission.
All electric motors have a power curve similar to I.C. The transmissions could be tuned to stay within the motors power curve for increased acceleration and run time. Mainly, you would want to stay within the most effecient part of the motors curve.
But we just race toy cars...