[BrianG]

This whole discussion is rather simple. For a given performance, it takes X amount of power. If using higher voltage along with the right kv motor, and geared appropriately, less current will be drawn.

As far as efficiency, that's simple too. Efficiency as a percent can be determined by (input_power/output_power) * 100. So, obviously, the closer the input and output power are, the more efficient a system is. Unfortunately, wires, FETs, etc all have resistance. That resistance * current = voltage drop. Then, that voltage drop * current = power lost. So, reduce the current to reduce voltage drop, and therefore power lost. There are other losses (ESC FET switching, Motor magnetic flux, etc) that are going to happen no matter what setup you use, but reducing current, using a high quality motor (even a few % points in eff can make a BIG difference), and making sure the vehicle is in proper working order (no drivetrain drag, bearings ok, etc) are the main things we can do to make the efficiency as high as possible.