Does rebuilding a hub increase efficiency?
For a typical rebuild the increase in efficiency is negligible. Think about the leverage you have over the bearings on a rotating wheel- it's pretty substantial compared to the friction of the bearings, even if they're not in perfect shape. The bearings would have to be destroyed for it to make a significant difference, and if the bearings are destroyed it's likely that the hub would be too, at which point a rebuild is moot.