Certainly simple enough to just set the split level to 1 and confirm the behavior 

I was under the impression that split level only death with folder splitting, not files too. I thought a split level of 2 should have kept all files, not folders, in the top level of American Horror Story.

This is probably the reason for the confusion.  The split level also applies to files that are within the limits allowed by the split level setting.  It sounds as if in your case you might be best of with a split level of 1 which would keep a whole TV series on the same disk?