By how much exactly can your cohort bring you down? I would say I'm in the top 3 of the cohort but the rest (well, most of them) aren't very hard workers and some of them didnt even do a section of the exam. I know that will affect my study score, but by how much? 
Not at all if the two above you are as good or better than you.
Damn. The boy ranked no1 said he stuffed up the exam and didn't even finish the paper whereas I thought I did a good job... That seriously sucks :\
Oh well, thanks for your help 
This doesn't matter - so long as there are two people in your cohort who did really well. For example, I'm just hoping that at least one person in my entire cohort of 250 got full marks in the exam (or close) haha. That's all that matters.
For chem I'd need a few people to do really well in the exam for my SAC scores to be good, as I'm ranked highly but not quite #1. Those people don't necessarily have to be higher than me. Say I'm ranked #5, if 5 people do extremely well my SACs will scale up to near full.