Don't do over 10 units, so many of my friends decided to forgo one of their units when hsc came along, that's not only wasted time for you but dragging other people down as well.
Yeah, I am. I dropped SDD last week to go down to 10 units including ext 2 maths. Looking at your situation, it seems like Chem is your least favourite subject and that you won't try as hard as the other subjects you are getting better in. Ultimately, this could waste valuable time that could be spent doing hw/revising/studying for your other subjects. So I say, drop it.
Ahh right now I'm doing 10 units as well with 4U maths. I know for sure I can handle it. Having another subject means you need to learn more, memorise more and as a result, have less time to study everything else. If you know you can do 4U, then do 10 units only