For Classic logs mana efficiency would be a useful metric since some classes can spam rank 1 healing spells and have superb mana efficiency whilst still doing well on the logs, but you can also have someone who’s just wasting mana and chugging pots so they can get a great log even if they’re being super inefficient.
It’d work a bit like the progress meter for guild runs in that it shows efficiency value over a fight’s duration, but instead of total group damage taken it would be healing / mana efficiency. I there’s already a way to do this on the logs to compare them all quickly then I’d like to know, it’s pretty easy to work out for individual casters, but if there were a tab to view it at a glance that’d be quite useful, since unlike retail there are lots of +mana things in classic and they change the healing landscape quite a lot.