I've got a 551. It's plenty fast/strong in a TC and takes a beating. Really good bit of gear.
Sadly superseded and the replacement 571 doesn't appear to do anything better apart from handling 7.4+V as you mentioned. I think it is a little cheaper though, so I guess that's a small win.
As far as I can determine, this is pointless in electric TC with current battery tech, we can't guarantee full 7.4V for a whole run (due to both peak draw and voltage drop), so performance of the servo would fluctuate.