I don't think those ratings should be taken too seriously. That is, difficulty is such a subjective matter that its basically impossible to rate music. I don't have any idea how the rachmaninoff prélude in c sharp minor is rated the same as Pictures at an Exhibition, or Chopin etude op10 #12. You can't even compare the difficulty in those works, yet they're all just "8". I mean, I think its a good idea to practice pieces from a broad range of difficulty levels instead of playing only "8+" pieces or whatever is at your limit. That way you can challenge yourself, but at hte same time feel good about accomplishing some easier pieces. Plus with piano in particular there is so much easier repetoire that is just as musically rewarding. If you want to think about it in terms of grades, then I would say it is a very good idea to go back for particular skills. For instance, maybe you have an extremely solid left hand and can play the op10 #12 or have finger agility for a schubert impromptu, but maybe you don't have the stamina for an all out 3 mvmt sonata or the finger indepence. I think its our ultimate goal to bring everything to as high a level as possible, but realistically, there are always certain skills that we have more of than others, and as such, we should always work on different levels to improve specific skills (and excercise certain skills). I also disagree with the grade system, in that, to a large degree how well you learn something is dependent on how intresteted and immersed in the work you become. Obviously at the higher levels this becomes less true, but I think to a large degree, the greater you can relate to a work, the greater chance you have of succeeding. So I wouldn't be completely daunted by a certain piece with a certain grade level. Give it a shot, and if its working out, then keep going, and if not, figure out what skills you need to acquire to make it work.