The voltage on the receiver does not effect the range. All the better receivers have a voltage regulator built in which limits the voltage to the rf section to about 3.8 volts (never measured the exact value) and that is the voltage the active part of the receiver sees as long as the voltage on the input is that high or higher. Increasing the voltage just causes the receiver electronics (the voltage regulator) to get hotter. Note that this does not limit the voltage the servos see as they work off of the unregulated voltage.