To answer your question, I'd put it at grade 9. This is a purely blind number, as I don't know what pieces are where in this grading system, but I'm guessing that 9 is appropriate. It isn't as horribly difficult as some people make it out to be, technically. It's challenge lies in keeping up concentration and endurance for 7-8 long minutes of repeating the same themes 3-5 times in a row.
In Canada, Beethoven's Moonlight Sonata is in Grade 5. But I have noticed that there is a very big difference between each individual movements. In the 3rd movement, the piece goes very fast and I believe it is over Grade 5. Can someone tell me what grade should moonlight be for the 3rd movement?