Many people have done it, but by going brushless. So you use an external ESC to drive the brushless motor, but also reduce the voltage going into the receiver. A diode is needed since most ESC give 5V so it's down to 4.3Volts. So your solution may be to follow the same route. Use an external brushed ESC instead of brushless. That would take care of providing safe voltage to the receiver and the servos. The big question will remain: Can those small brushed motors be driven by a higher voltage, and if so, for how long? Again, safe bet for more speed is going brushless.