We have tested this as we were wondering as well. We have three "designs" Storing metadata as individual tags, one using a metadata document tag, and one using a metadata property on a tag.
This test was done making 100,000 parent tags. Using individual tags used between 1.5 to 2 GB as a baseline. Using the other two designs used quite a bit less memory, 780 MB and 700 MB respectively. I think having a separate document tag would be the best way to tackle this design.