I would suggest at least a #14 wire. It's like trying to put too much water pressure through too small a pipe. Use a bigger pipe requires less pressure to get the same amout of water through in the same period of time. If the wire heats up, like your motor, the resistance of the wire increases with temperature. So the wire ends up using some of the voltage. Like trying to make water roll uphill. If you run stock or 19T, use #14. If you run open mod, use #12. Only because the bigger wire is also heavier and weight eats up horsepower that you can't afford to loose in the stock classes. If the wire heats up and makes more resistance then you loose current flow to the wire. Just like there is a voltage drop across the servo, there is a voltage drop across the wire, and that means less volts to the motor. The is no power in the wire, only current flow. Power is the work that is performed by the motor.