Collection per content type
My question got answered on drupal.org:


  Posted by marcingy on August 16, 2013 at 7:03pm:
  
  
    This is mirroring how core storage works. fields_current.node contains the data for the current revision. There is a collection per entity type which is the smallest division in core in terms of storage.
  


I guess I will have to build in the flexibility myself :-)