My personal experience with n-back mainly revolves around discovering the large variation in my ability to focus. Practisibg does get me better at the n-back task (not anything else as far as I can tell) but daily form determines results to a large degree. This effect seems to transfer somewhat: on weeks I'm overall worse at focusing on n-back I appear to also produce fewer commits.

But what determines this daily form I don't know. I suspect sleep, stress, and exercise has something to do with it.