If like me you have a lot of Motion Template and don't want to uninstall but want to clean your list in FCPX, try FCPX Manager, this is the app I have decided to write to resolve this issue and that will allow you to Enable or Disable any FX, Generators, Title or Transition. And if you need to re-enable all Fx for a project, just export as XML and load the XML in FCPX Manager, you can then re-enable all what you need for this project.To help you manage your plugins, you can also add notes, categories, url, ranking and filter your database ... easy to find this particular effect you have previously parked ! Disabling Fx will not only clean the interface but will reduce FCPX Startup time.
I have tested with Fx from various companies like FxFactory, Pixel Films, ...
You can also create Favorite folders with automatic copies of your preferred plugins to have easy access in FCPX.
I am working on a new version to fix the issue of broken Fx in FCPX when using project from other computer that don't have the same path in the Motion Template folder for Fx.Any one that want to give it a try, look at fcpxmanager.com