Beware that servos are the limiting point for the RX battery voltage. Most receivers can handle 7.4 volts, but not the servos. Most servos are limited to 6 volt operation. There are a few servos that just came out that can take 7.4 volts, but are very expensive and super high performance. You could use a 7.4 volt battery with a voltage regulator, but now you are adding cost and complexity.