There's no guaranteed benefit from doing more units as in the end only your best 10 units with 2 units of English are used to calculate your atar.
Doing 12 units might mean you perform lower overall because you wouldn't be spending the same amount of time studying each subject as a person with 10 units could.
However, in saying that if you are consistently performing well in all the subjects it might be an advantage since in the hsc if you screw up one external exam on the day it wont negatively affect your atar.
Overall it is better to do 10 or 11 units units, and if there is a clear subject that you are underperforming in than the others while doing 12 units you might as well drop it since it won't count and will use up time you could use to study th other subjects.