Do the HS-805BB and HS-755HB servos have the same construction as the HS-422 where the control board inside is connected to the motor and pot with wires instead of being directly soldered to the board? Also as a followup question, do the Sabertooth motor controllers offer variable speed motor control?
Unfortunately, I haven’t opened either of those servos lately. 
I do know that the Sabertooth motor controllers do offer proportional control.