All GWS Rx already build in regulator 3.3V ( GWS has 3V regulator for PG-03 Piezo gyro ). Technically said, you can use sigle cell Li-ion to fly. But all servo need to redesign for getting better power out put at 3V. But need to design match gear drive at low current not to make Li-ion battery's Vcc drop too much.
Trandtional servo will fall to sleep under 3.5V, but GWS is working on a new circuit for 2.8V ( Sorry Pico servo is too small to put this new PCB into her case!
C-mos circuit can even operated at 2V, but high frequency RF end will get very bad performance, since dynamic range become poor. So, we'd better keep it use a 3~3.3V regulator for Rx.