Dude I would drop Physics and Chemistry. Your strength obviously lies in the written and spoken word. Don't drop Society and Culture if your ranked 1.
If you drop physics and chem, you'll have a lot more time for studying maths. You might consider picking up English Ext 2, as well as History extension.
Seriously, 15 units is ridiculous and I think it's stupid any school would allow it!
If you were at uni, and you had 5 subjects per semester, would you do 6?
IT's STUPID how people say 'Oh i'm going to have these extra subjects as backups' -> WHY!? The HSC is a time game, and only 10 units count, doing 15 is crazy because 5 units, which is sometimes 50% of a person's HSC, wouldn't even count.