I don't think you are kidding yourself.
On the scale of things in life, (things way bigger than the HSC), should you expect to drop everything in life when big things come along? Will you have the same mentality at uni? Is sacrificing sleep/good food/exercise at any point in life a wise choice, from academic drive to having to care for sick relatives to any big other stressful time in your life.
You may as well try to learn to balance now, it may come in useful in uni and may come in essential when you enter the big wide world out there. I can absolutely guarantee I know many people who did the exceptionally well who did so much in HSC year. I know 99.5+ers that went overseas for a few weeks of yr 12. I know people who received 95+ with extreme pressures of caring for sick relatives, working massive hours at a job out of necessity, health issues etc.
In yr12(and basically any other life stage), there is the possibility for people to manage, at varying levels;
Sleep, food, exercise, good results, social life, family responsibility and other extracurricular activities. You just have to prioritise. I couldn't keep up a 20 hr dance load once I hit the HSC, also coupled with going overseas in yr 12 and family responsibility. So I scaled back my dancing a bit, but I didn't completely cut things out of my life. Everything in moderation.