By high, how high roughly? I have a very strong cohort and our SACs are generally much harder than VCAA exams.
This is something that is really hard to judge, and is quite dependent on things such as the difficulty of the exam and how your classmates perform on the exam. My general rule is to simply try the best that you can (because really there's no beating that, right?), and hopefully receive a pleasant surprise at the end.
That being said, the exam is indeed worth a high portion of your overall study score compared to other subjects, so your SACs, and hence ranking will be less important here (since you keep your exam mark, but your SAC marks can change). But, considering you mentioned having a very strong cohort, I would say that if you were (and are prepared to work to) maintain a rank which is amongst the top students (I won't mention a particular number - I feel it is irrelevant and quite difficult to give one) you will be in good stead to achieve your goal (if not higher).
I'm not sure how you feel you fit in terms of your cohort, but if you can maintain a high average (with respect to others) like you mentioned, even if you are not in the top, say 5, 10 students, you can still achieve a high study score. Further to this, considering the fact that your SACs are relatively difficult, as well as your confidence in being able to do well on the exams, plus your strong cohort, I really don't think you have to worry too much
