Well, yes, obviously, if he sets the voltage up to 6v then it will be a huge increase in performance.
Still, you can't go wrong with more torque. Is it needed? no. Is it better? yes Remember, the sf-20 is also pretty slow, so a high torque upgrade will most likely increase the speed also...
Small or big savage, either way the sf 20 is a weak servo, and more torque is always good, you just need to set the endpoints right so you don't tear stuff up.
It will vary slightly on every engine. And the only time you want to pinch the fuel line to shut it off is if you're having trouble tuning and you need a general idea where your lsn is at. It makes the engine run lean.
Lean = bad
sf-20 servos are weak.......
Pretty much anything else with metal gears will be better. Hitec, futaba, savox, Get something with a lot of torque, but not too much (400oz in is NOT needed)