So not looking forward to next year... 
I don't really understand about the ranking in a cohort, how does that affect anything?
Firstly, the mark you get on the SACs do not matter. For example, if you get 99% in a Chem SAC and everyone else in your Chem cohort gets 100%. So even though a 99% is an excellent SAC mark, however relative to the Chem cohort, it isn't because you are ranked last.
But what if I am ranked first? How does that affect my study score?
Well, if you are ranked first in all the SACs for a particular subject, then as you know SAC marks are moderated by VCAA based on how well your cohort does in the exam(s). So if you are ranked first in the SACs, then you will get the highest moderated SAC mark. The person ranked second will get the second highest moderated SAC mark?
Does this change my exam mark?
No, your exam mark will stay the same. The exams change your SAC marks (if permissible), not vice versa.