Tell me if I'm wrong.

HV servos are supposed to be able to handle a 2S lipo when fully charged..which is 8,4v

So can I use 8,4v permanently to power my HV 1268sg savox servo via my external BEC without degrading its longevity?

Same question for the receiver

it should be just fine. ive never tried running them at 8.4v however i cant see it being an issue. in my crawler i run my servos that are rated at 6v at 6.8v without issues. the way i look at it is straight from a fully charged 5 cell receiver pack is over 7v.

Suggest checking for the servo heating up unusually. If I’m not mistaken some have experienced a fair bit of heat when pushing the voltage up, at some point that can’t be good. All else being equal more voltage likely means more current and more heat.

Ok, is there any way to calculate the gains in speed and torque going up in voltage?

I know some guys are running their 7955TG at 7,4v even if it's supposed to be a 6v servo. I'll check the heat then and see how it goes, I just hope the receiver will be fine

The 7955tg is a special case since the manufacturer has said 7.2 is ok. In general i i wouldn't recommend it. If it's meant for 7.2 i would just run it there.