So if you get the highest exam score you automatically get the highest sac score 
Actually, if you get the highest exam score you DON'T automatically get the highest sac score. Your sac score will be based around your cohort ranking. If you are rank 1, then your exam score (if the highest) becomes your sac score. However, if you are not rank 1, then you get another person's exam score as your sac score.
So basically, SACs DO MATTER!!
what i was trying to say is that you don't need to be number 1 in sacs, and that you should focus on your unit 3 exams as opposed to your upcoming sacs.
And i'm pretty sure that your sacs score do not moderate your exam score it just means that a higher exam scorer will have a higher sac mark in your cohort. For example
Student 1 gets 100% on unit 3 sacs and 85/90 on exam
Student 2 gets 90% on unit 3 sacs and 87/90
Then student 2 will be achieve the higher study score, because his exam score will reflect on his sac scores