Proudwinner,
It depends a little on what servo it is you are going to use? If you check the brand(airtronics, futaba etc) and look on the servo chart on their website it gives you specs on how the servo will operate at different voltages(such as speed and torque). You should deffenetly call keyence and see what they say, i believe that most esc's operate servos at 6 volts. There might not be a problem with your servo but it possibly will move slower and have less torque than it could.
Blake