RE: 4.8V versus 6V for receiver battery?
Keep in mind also that our servos are going to draw what they want, not what's available. So if they don't want it, it doesn't matter if it's there or not.
If they already have more power from the 6V, they're not going to be asking for more juice.
But if they need more power, they're going to try to draw more juice and with 5 cells they're going to have more available.
But if they want more than what's available, they simply do not get it.