Scope for improvement
(There might already be a standard term for this that I don't know about.)
By scope for improvement of a skill or learning technique, I mean something like the difference between "how much a person highly skilled in this technique (i.e. a virtuoso user) gets out of it" versus "how much a novice at this technique gets out of it".
It's sort of related to terms like "high variance" and "heavy-tailed".
"I'm particularly grateful to Andy Matuschak for many thoughtful and enjoyable conversations, and especially for pointing out how unusual is the view that Anki can be a virtuoso skill for understanding, not just a means of remembering facts."