Well, if Final Cut creates a copy of the effect when you use it, how come in the past when I renamed an effect in the Motion Templates folder, in Final Cut, the effect showed up as missing?

That said, that gave me exactly the idea to solve it! 🙂
In essence, here’s what’s happening:
1. Final Cut makes a copy of the effect, some sort of cache maybe? (clearly it’s doing something, because it’s rendering the old version)
2. Final Cut tries to read the effect from the Motion Templates folder anyway.
3. If the file is there, it doesn’t bother updating from the file (So, um, why was it trying to read it? lol). If the file isn’t there, rather than fall back on the cache, it fails. (So, why is the cache even there?).
This behavior is the key. It FAILS if the file isn’t there. Therefore, here’s the sequence to force an update:
1. Rename the template (add a “-renamed” or something to the end of the folder containing it
2. Launch FCPX and load the project. You’ll see missing effects panels.
3. Quit FCPX, rename the folder back to the original name
4. Launch FCPX and load the project again. Presto, effect updated!
I’d like to see this become easier, as immediately previewing the “effect” (pun intended) of changes like this could be extremely useful at times. Still, good to know it’s at least possible to get it to happen, even if it is a bit “workaroundish.”
Thanks for the help! 🙂