What is the impact of deleting render files of an archived project when portions of it have to be copy/pasted onto another project?

Render files only make editing easier. Otherwise, you don't need them for anything. Delete all your render files from time to time to make room on your hard drives, especially for projects you have already finished. For projects you are still working on, you have the option to delete only the unused render files. Active renders will remain intact.

While you edit in the storyline, you will probably want to render the files used from time to time (if you don't have automatic background rendering on.) Other than that, there is really no use for render files. In finished projects, they are excess baggage, not used for anything and not useful in any other type of project.

You can delete the project render files from any project by selecting the project and in the File menu, select Delete Render Files. You also have the option to Delete Optimized Media and Delete Proxy Media. If you are finished with a project, these are all good methods of recovering significant disk space. However, deleting render files is never a bad idea. If worse comes to worse, they can always be regenerated within any project storyline. It won't take any longer than having them rendered in the first place.

In further context, every time you make an edit, add a title, effect, transition or generator and render, FCPX will create a new batch of renderings for that new "state" of your project. FCPX has "unlimited undo" -- that's a lot of render files!

Render files have no effect on the editable media imported into your Events. Even if you delete Optimized and Proxy media, you will still have access to the original media you have imported.

Right!