Something else to keep in mind is motor efficiency. DC brushless motors run at optimal efficiency (ie. highest mechanical power output for least electrical power input) when copper and iron losses are balanced. Copper losses increase as current increases while iron losses increase as motor RPM increases. Spinning a motor too slowly and over gearing can cause copper losses to be too high resulting in an efficiency loss. Spinning a motor too fast and under gearing can cause iron losses to be too high resulting in an efficiency loss.

For modeling purposes where the motor's energy is being transferred into kinetic energy, weight is a factor as well. Running a motor too big for the power required would be inefficient because the model will not benefit from the full power potential of the motor and so a larger percentage of the power produced is consumed in accelerating the mass of the motor. Running a motor too small will result in copper and/or iron losses to increase too far because of the model's high demand causing temperatures to run away.

Getting the most efficient system is really a matter of matching the motor, speedo, and ESC all together for the model in question given weight, gearing constraints, power requirements, and battery space.

BTW, I've read many references here to 30000 RPM being an optimal motor RPM. From where is this number born? Is this specific to Lehner motors? Or is it a number based on common car gearing?