I'm using the Savox 1256 for steering & 1258 for throttle/brake duties, been using them around 2 yrs & running them on LiFe with no problems at all & the extra little bit of voltage (.6v) makes the servo slightly faster with a little more torque.
The set up I use does the job just fine & will for alot of people, I'll go with a 1256 for throttle next time as well, just for the extra braking force if needed, throttle doesn't need a super fast servo. Brother is using 2x 1256tg servo's on a LiFe pack & so far its been working great, throttle/brake speed & torque is plenty.