The sweet spot for power is 3400 RPM. That's the point where voltage at the motor = pack voltage and current has not yet dropped because of back EMF.
A rough diagram:
66 Volts is usually what the pack sags to under full power.
Amps is proportional to torque.
Volts is the "pressure" required to push the amps in, and this value is proportional to RPM thanks to back EMF.
What does this mean for efficiency? I have no idea.