Well I do Musicianship but I'm guessing it's a similar situation to Theory...
Nobody does Grade 8. I don't know why, they all stop at Grade 7. There is a Grade 8 for Musicianship, but nobody seems to do it...I really enjoy Musicianship and plan to continue on with Grade 8 after the HSC finishes, with views to possibly going for the AMusA.
I've never heard of anybody doing AMusA Theory or Musicianship, so I assume they're not very popular.