Great question.
I had a private student last year who didn't do too well in his unit 3/4 SACs because he didn't work hard enough and honestly art was his passion (which I told him to pursue!!), however, he actually surprised me and did really well in the exam (A+), and ended up getting a 38, so yes the exam can make a massive difference.
I tell all students this, however: DO NOT WORRY ABOUT HOW YOU'RE GOING. Just do, learn and focus on excelling. If you get caught up in what's gone wrong it consumes your thoughts to the point where you actually forget how to excel in the first place. Be an independent thinker and THINK WHY certain linguistic features occur - when I was in year 12, my teacher was very lazy and so I'd go home at night and research myself. I would think and 'ponder' (sorry for the lame word) why certain processes occur in language. So many students get caught up in just memorising what they have to know for a SAC that they forget that to excel in a subject you must have conceptual understanding of it. I was shocked when I asked students, what is the difference between informal and formal language and how would you define it. Most students can't, but I have found this also stems from the teachers not making it that simple for students in their classes.
I hope that helps - it's not a rant, just my viewpoint and how I believe you can succeed in not just E.Lang but any other subject!
Thanks!
Dmitri.