I would say that it has an effect on your study score. I got a higher English exam score than some others at my school, but they got higher study scores than me because they were ranked higher. SAC percentages don't really mean much. The standard varies from school to school, so it isn't really much of an indicator. What matters is your cohort's exam performance. Yes, shit exam performance will let you down, unless you're ranked first within your cohort (if you come from a weak school). Obviously if you have a strong cohort, the bulk of the students will perform well in exams, so your SACs won't be moderated down as much, if at all (it might even go higher). Unfortunately, the only SAC marks that weren't moderated down for me came from the subject I finished first in.