The GF-E works with both S-FHSS and regular FHSS so it will be a great choice. I acutally use one in my buggy.

Savox servos have issues by drawing too much power - more than the ESC's BEC can output. The actual receiver has little to do with when those cause brownouts as the receiver just passes the power through but some brands are more prone to it than others, i.e. with Spektrum radios it can be a big issue, Futaba is known to be one of the more "Savox-resistant" brands. Basically, the less current/voltage level the receiver needs to maintain signal means there will is a higher probability the signal will be maintained when the Savox is feeding like a beast without having to add a glitch-buster or external BEC.

I wouldn't upgrade to FASST ever - the receivers cost too much. However, if you think you are getting perceptible lag on FHSS, an upgrade to T/S-FHSS like the 4PL(S) uses will get you to within a couple milliseconds of FASST specs for way less cost.