I think it depends on how strng your school is and what your specific ranking is within those classes and not so much your grade. Like idealy, imagine every school was equal, 1000 students did english and to get 40 or above, that is top 9%. Then lets assume their are 45 corhorts/schools doing the subjects, that means that only 90 kids will get 40 or above, and idealy that means that the top 2 kids in each school would get 40 or above in the subject. Obviously not every cohort is equal, some may get higher in sacs due to school so the exam acts as a way to balence sac scores which depends on how your cohort does on their sas compared to exams.
^^^the above is really inaccurate due to a ridiculous amount of assumptions but it is meant to illustrate the importance of ur ranking even more than your scores. If ur school gives super hard SACS where the top score is 60, but then on the exam the average score in 99% for ur school, then those SAC marks go wheeeeeee all the way up, but they will go up so that the ranking doesn't change much, so if ur rank 1, lucky u I guess but if u flop the exam then ur mark will get dragged down soooo in short, doing well on the exam matters, don't stress too much about ATAR, it is possible and good luck. Also remember some special consideration is being given to students so apply for it, who knows may increase chance of getting into course?