The HSC results you get, I've found, are often proportional to the amount of work you do.
An ATAR of 87 corresponds approximately to straight Band 5's, so they do require some work, though obviously not as much as what you would need to do to get a band 6.
(I'm not trying to say you shouldn't aim for 6's, I'm just saying you should look at how much work it is, not how hard it is. Obviously other subjects (4 unit maths etc etc) are more difficult than others, though the same rule applies).
HSC was a huge anti climax imo
this and this.
I haven't read the whole thread so sorry if I'm repeating anything already said, but I really wanted to reply after reading the thread title...
HSC isn't difficult. Each year of high school gradually increases in difficulty (generally), and Year 11 and 12 are no different. I think if you're naturally intelligent, you can get by on very little study/effort.
But in general - you don't have to be a genius to do well.
Like jetblack said - effort = results.
It's a memory game (maybe not in maths...), and in a sense it's a test of mental strength (i.e. how well you deal with the constant pressure)
HSC is a massive nothing. Having just finished, I learnt how truly meaningless it is the very second you finish your last exam. Ok, you need it to get into future education pathways, but there are clearly many alternatives and "side doors for entry" as I've been told so many times.
But still do your best - it's fantastic practise, and you learn some invaluable coping/studying mechanisms for later in life.
Just don't go stabbing yourself in the eyes with a pencil, it's not THAT big a deal!
Best of luck with your HSC =)