Hi Snow Leopard,
I am a current VCE English student and I can only assume that it the extended length of your SACs would negatively impact your ability to churn out an essay per hour in the exam. Giving that this is the case and the fact that your cohort is weak, just try to aim for rank 1 in your SACs and you will be fine. I'd suggest you aim to practice your essays under 1hr time restrictions regardless of how your SACs are laid out because doing so would only enhance your performance at the end of the year. My school for instance, gives 1 hr time limits for all SACs (plus additional reading time if needed) and i came from a year 11 English SAC structure in which allocated more writing time to SACs (75-80 minutes). Therefore, i struggled to finish essays in 60 minutes, however after practice, I managed it overtime.
Don't know if i am the most qualified person to answer your question but that is my personal point of view.