Effects on motor encoder by over-volting
Increasing the motor voltage from 12 to 18 volts will increase the speed proportionally from 7650 RPM to 11475 RPM. Note that 12 volts is listed as the nominal voltage and 11900 is the maximum speed. That would indicate that 18 volts is probably acceptable for the motor. The maximum speed of the encoder is listed as 37500 RPM, so that should be ok also. The lifetime of the motor brushes and bearings and the encoder bearings will be longer at the lower speed, but the lifetime at the higher speed is apparently estimated to be acceptable by the manufacturer.