You don't need to be a genius to achieve a UAI of 90+, I'm not one: when I first tried SAM I got a UAI of 95.55 and I average only high eighties in my school (highly competitive private school). UAIs of 90+ are definitely not out of the question, but dependng on what school you go to, the very minimum raw marks you need would be mid 80s. Of course, the worse your school performs, the higher your raw marks will need to be.
Don't think about scaling: even if it's true, you should be choosing the subjects you love. Try and improve your marks as much as possible between now and your HSC (some people have moved up a whole band
), and you won't need to worry about anything.
If you're worried about your raw marks, seek advice on this board and your teachers and past HSC candidates. My advice: set yourself crazily high goals, but not so high that's impossible. When I finished SC, maths mark was 88. Now, in my first 2U and 3U Maths assessments (Year 12 now), I got 29/30. I kept on saying to myself, I'll top the grade, and get full marks, even if it's only maths. I thought it was crazy, that I was setting myself up for failure, but dreams can come true, and I'm only a mark away from it.
Good luck!