I'm planning to finish the syllabus for all my subjects by the end of the year (going to work on all my subjects in the end-of-year summer break). That way in my classes, my teachers are pretty much going over what I've already learnt myself and I can ask questions about concepts i don't understand.
I'm currently undertaking 13 units for my HSC course. I would say don't drop anything as of yet because you don't really know how well you're going to do with the HSC syllabus being implemented upon your subjects, considering that HSC and Prelim work are completely different.
SO, keep them all...