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