Specs are something like this:
K79 up to 5.5
K86 up to 6.0
Haven't tried K79, but I know that light sailors (65kg) sometimes use K86 with a 6.5 (I can't @ 86kg, I've tried). Following this, my guess is that you'll gain no early planing, but worse handling with a 5.9. Compare the ride with your 5.2 and 5.9 and see for yourself.
From other sailor's reports I got the impression that Port Phillip Bay can fire more than 25kts now and then, so it might be a good idea to go for a K79/80, rather than a K86. Ask around what other locals are using.
Last edited by Screamer; 22nd May 2009 at 02:25 PM.