git subtree as a solution to partial cloning?

I am considering different ways to get git to handle repositories with
very big files in a setup where partial clone is required, and it
seems git subtree might be a part of the solution.

Does git subtree support splitting at the file level, or only at
directory level? Also, how are conflicts handled when you subtree
merge changes back to the master? For this to work in practice, I
suppose the users of the split repositories should see the conflicts
and fix them themselves. Can the reduced split repositories reuse pack
files from the original repository? Can you think of any other
limitations to git subtree that would prevent it from working with big
files to support a partial cloning setup?

The alternative seems to be git sparse checkout extended with
non-existing narrow clone, but it seems that a git subtree based
approach might be simpler.

Regards,
Asger Ottar Alstrup