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".
Thinking in terms of scope of improvement encourages looking at techniques in terms of "What can I get out of this if I master this technique?" rather than "How much did the average participant in this study gain out of this technique?".
There is a distinction between interpersonal skill level differences (how good is the best person at the technique compared to you) and intrapersonal differences (how much can you improve if you practice). The differences are related because interpersonal differences provide evidence for potential intrapersonal differences. Strictly speaking, scope for improvement (in the sense I am thinking about) is only about the intrapersonal difference, or rather the interpersonal average of intrapersonal differences.
- Spaced repetition seems like a learning technique with large scope for improvement. My impression is that most people use it for things like learning foreign language vocabulary and other "isolated facts" they want to memorize, and conclude that there is little scope for improvement. What they don't do is look at the "masters of spaced repetition" and try to cultivate the skill of using a spaced repetition program for understanding. Michael Nielsen: "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." 
- Probably many mundane skills like the skill of counting has limited scope for improvement. Most people can't learn to count many times faster or many times more accurately or to very large numbers. (At that point, they would probably want to make use of a counting device that just supports an increment operation.)