Mac33,
Since you've got a 370 cm mast already and live in Australia, you can get a 3 cambered Severne Overdrive 4.5 m2 slalom sail for example:
http://2009.severnesails.com/2009/pr..._overdrive.php
Or get a 2008 Severne 4 cambered Code Red 4.6 m2 sail (cheap close-out sales or second-hand):
http://2008.severnesails.com/
In very gusty conditions, a full-on cambered sail is much more stable under load than a no-cam wave sail. You'll be faster, keep full speed longer and cope a better balance with the unavoidable sudden gusts and lulls on your dedicated slalom board.
Cheers!
JM