@roy, true hsc eng studies is difficult because the stuff taught is unlike any other subject, but as long as you pay attention and study hard you'll do fine.
also it doesnt really matter if 1/5 of hsc eng studies is aero.
the first year for all engineering streams at unsw is the same.
the mechanical and manufacturing faculty of eng (covers mechanical, mechatronic, aeronautical, manufacturing, and naval eng) do the exact same subjects for the first 2 years of uni.
hsc engineering studies teaches the absolute basics of materials science, mechanics and report writing, all of which are studied in all 4 years of your degree.
being close to the end of my mech degree, i strongly believe that doing engineering studies for hsc will be beneficial for you if you want to undertake an engineering degree after the hsc.