RE: 4.8V versus 6V for receiver battery?
Just remember that Power (Watts) = Voltage X Current. The more torque you have, the more power it consumes.
If you use more power, it has to come from somewhere. Increase the voltage, and keep the current the same, then the watts, or power consumed, goes up. So, going from 4.8 volts to 6 volts satisfies that equation requirement. Current may just stay the same, but the voltage has been increased to 6 volts so there is more power available. But there are more variables in this than a simple math equation. Heat is one. You may not feel it, but it's still there and is a direct result of power consumed and consumes power to create heat.
Simple fact is that if you use a 6 volt battery, the servo acts faster and most of that is from the increase in the "electromotive force" or the "voltage" increase of 1.2 volts from 4.8 to 6 volts.
Now with that said, there is a good reason to increase your capacity from, say 900 mah to 1200 mah and that's two fold. To increase the flight time AND/OR to provide the necessary increase in current available for higher torque and more receiver current.
If you take a system with five servo's, change the battery from 4.8 volts to 6 volts, but the same capacity, chances are that they will last just about the same lenght of time before requiring recharge. And, as I said, there will be other current draw issues such as heat (more power, more heat) which will also consume power which may reduce that flight time slightly.
Best thing to do is monitor your battery condition before and after each flight, especially before. If it's borderline..charge it.