For throttle I use cheap light weight standard servos like Futaba S3004 or equivalent.

I'm not aware of a reason why one would use an expensive, heavy, high torque digital servo on the throttle unless you have a terrible installation with lots of drag on the push rod or you forgot to disconnect that powerful return spring from the carb throttle lever.

As for speed, don't you find that servo response on 6 volts is quicker than the engine can respond?