To be clear, the Futaba FASST system is not voltage sensitive. It will remain linked and functioning until the voltage is too low for the servos to work correctly. Using digital servos that point is 3 volts, which is the minimum voltage most digitals need. Analog servos will go lower. There have been reports (I cannot find the link right now) showing the FASST RX remaining linked as low as 1 volt.

Futaba recommnds using a battery that will deliver sufficient voltage under the load you will use.