I recommend text storage type.
You should be able to "upgrade" the existing items to the text storage type since no data is stored in the file/content field anyway.
The revisions are designed as a full audit trail for the assessment to see when it was changed and to track answers to each revision of a question etc.
So you need to consider that very carefully before changing it to make sure you get consistent data back out of it.
You could create items that are not content_revisions at all that are just stored in a custom table. There should be documentation for that in the acs-content-repository documentation but it might be more effort than it is worth. Removing the "file" storage type would probably help and be easier than rewriting every query in assessment.
