Thank you for this most interesting post. This is a subject that interests me. I can suggest four items:
1. Hinson’s famous book on piano literature grades each piece in four categories, easy, intermediate, medium difficult and difficult. It is pretty comprehensive.
2. Jane McGrath’s book grades most pieces on levels 1 to 10. It is intended, I think, for teaching purposes because many difficult pieces (e.g. Godowsky) are not cited, and even pieces which I would consider to be of only medium difficulty attract the top grade of 10 (e.g. Handel’s Suite number 8 in F minor).
3. Klaus Wolters’ Handbuch Der Klavierliteratur Zu Zwei Handen and Georgii Walter’s Klaviermusik: one of these (which I am not sure) has a grading system also, referred to by Hinson in his preface.
4. The syllabuses of the examinations of the Associated Board of the Royal College of Music (ABRSM), at http://www.abrsm.org/.
Now I wonder if you can do me and the other readers of this forum a favor, please? That would be to post on the forum the syllabus of the Royal Conservatory of Music (Toronto). I have looked on the web site of the RCM but they do not list the piano syllabus there. Even if you listed only the repertoire for, say, the Grade 10 and the ARCT, that would be a great service for us all.
Thanks, and best wishes,