No, the caches don't overlap - they are strictly separated for each
configuration:
/var/lib/mock/root-cache/<config_root>/{clean,linked}-cache-copy
So you can have builds simultaneously (i.e C-4, C-5 and FC-8).

And yes, linking is much faster then copying or extracting an
uncompressed tarball.

The only cost of the system is only linking and replacing dirty
linked files with rsync that may be changed from previous build
operations.

Please keep in mind, that the link count of the base files in the
build root are only decreased instead of the files are deleted
when a 'mock clean' is made. So not only the restore from cache
operation is faster, also the cleaning one is.