its not a matter of scaling, its if the subjects are for YOU. Your teachers will say to do the subjects you enjoy, and i advise you take this piece of advice seriously. You will see exactly what this advice means come trial and post HSC. HSC courses arent only there for your uai, you take the knowledge with you outside of school, and it will define who you are and your university choices.
Ask yourself:
1. are you good at these subjects?
2. Do you enjoy them?
3. Are you being realistic and doing subjects taht you can handle?
Too many times i have known people who choose subjects purely on scaling, and bomb out. Trust me, Maths ext 2 was not for me, and by term 3, term 2 even, it had taken its toll. It was so hard getting myself to push through the finish line with ext 1 and 2 maths on my belt. I was lucky that the mark i DID get scaled into the 90's. But it was hard, dont try to make anything more difficult for yourself than it needs to be.
On the otehr hand, year 12 is also a time to raise the bar and set challenges. Sometimes the harder you push, the faster you will roll. Try 2-4 units of subjects you think will find challenging, but also you take interest in. BUT, id advise not doing anything too different from the preliminary hsc, or it will be VERY hard.