On the other hand, many of those tags aren't used as search-metadata but as content-metadata, and those are often the accounts with thousands of them. Perhaps a checkbox, on import, for "yes try to map the tags as tags" or "no, include all tags as text footers to the entry content" with the caveat that after a certain processing limit, all tags will become text footers, and/or the opportunity to add a unique tag to the import so the entries can be found again easily.