Coming from h16 you are probably a bit obsessive about mast rake! As long as the F18 feels balanced, moving the mast about doesn't make much perfomance difference.
The helmsman uses the downhaul as a coarse setting to keep gust control within the crews armful. if you feel that he is being over-worked - pull it on a bit. if you think he is having an easy ride and nodding off - ease it a bit!