There are no penalties as regard your UAI if you drop a subject, the reason many people have additional units to their required subject load is that they may desire to have a "safety net" of sorts in case they do poorly in a subject or it may be to follow personal interests, or because they wish to prepare for certain courses at university. If none of those reasons apply to you, and you have your 10 required HSC units(including 2 units of english) in four subjects and you completed 12 prelim units in year 11, then there would be no effect on your UAI or substantial reason to keep a subject.