Hey guys, I'm back after a tumultuous HSC ride. Thanks for all your suggestions, although to be honest I was just looking for one person who thought 13 units was doable, and then stick with it. Which is what happened. At the time, I met an overwhelming response from almost everyone with the authority to advise, against doing 13 units. My take is that the tables do turn in year 12, it doesn't mean that if you don't like a subject then you won't do well so be careful before dropping (I didn't fancy chemistry, but it turned out to be my best subject).
I got band 6 for all those subjects. I studied for 1-2 hours each school day, 4-6 hours each weekend and 5-6 hours each holiday day if you're wondering how much work it took, which basically means that I got more free time (plenty for commitments such as extra-curriculars) than study time. The time you take off right before HSC is also going to be more time to study than you think, provided you sort everything out before trials.
Like all advice, you should take this with a grain of salt because what do I know about everyone else? For those of you wondering, it's possible to get 99.95 and very likely to get 99+ with a 13 unit (and even 14 unit) workload so quantity of study time on each subject is definitely not quality.
Wish future HSC takers all the very best in what is now still quite a screwed up curriculum.