There is another kind of useful-but-not-valued, and that's the person who does things that management ignores, or finds not worthy of investment of time, but inoculate against future trouble or enable other work, or make it considerably easier, down the line.
If you engage in that kind of work, it's not just that you'll not be recognized/valued, but rather - be thought negatively of, for doing that.
> "this will improve our efficiency in performing Y by 80%"
Even if we took your example: Suppose it will improve the efficiency based on other, additional, work that would take a long while, and it is not trivial to understand why.
> "help us achieve our goal by reducing time wasted on Z."
Manager may well tell you:
* "I never allocate time for Z, only for concrete project goals. And nobody ever complains about wasting time on Z."
* "I don't know if you guys waste time on Z or not, but I definitely know that if I approve your request you will be wasting time on it, rather than implementing a new feature or fixing a bug."
If you engage in that kind of work, it's not just that you'll not be recognized/valued, but rather - be thought negatively of, for doing that.