Document metadata is stored only in the SQL database, so restoring volumes is not necessary to recover this information.  What you do have to consider is that if you restore your database to that point in time without restoring the volumes, there's potential for inconsistencies.  For instance, if a document was deleted your SQL restore will undo the delete from the index but the pages won't exist on the volume, or if a document was moved between volumes the database will point to the old location (where it no longer resides).  Of course, if there were no changes on a particular volume since the field shortening you don't need to restore that volume.