SACs are more for your ranking than anything else. IMO, they really matter in that respect. As all schools have different levels of difficulty in SACs, their results are more or less moderated in the attempt to make things fairer across the board. This process involves looking at the rankings of those within the school and how these reflect the exam scores achieved by the same group. In the majority of subjects which have a 50 SAC : 50 exam kind of weighting at least, it works
something like this:
School A:
Tom gets 83% in his SACs (across the year), and is ranked #1 in his class. Scores 100% on the exam. VCAA sees that Tom's school was marked hard against VCAA's exam standard and scales his SAC results somewhere closer to 100%.
School B:
Ben gets 92% on SACs, ranked numero uno. Flunks the exam though, 84%, which happens to be ranked #3 exam result at the school.
Loz gets SACs: ranked #2 at 87%. Exam: 91% (#2)
Jane gets 65% on SACs, ranked #3. Really picks up before the exam and manages to pull a 95% on it (#1).
When results come out, all of them are awarded their result for their exam, however their SAC results are 'scaled' accordingly.
Ben: Exam - 84%. However, as he was top ranked in SACs, his SAC results are moderated according to the top EXAM result, regardless of the fact that he didn't actually achieve it. So his SACs go from 92% to ~95%.
Loz: Exam - 91%, SACs ~91%
Jane: Exam - 95%, SACs ~84%
If you wanna be schneaky, if you're ranked #1 at the end of the year, then you can help everyone else do as well as possible on the exam

. Not sure how effective that is in practice haha.
Anyway
as a rule of thumb, it's more or less #1 SAC scaled by #1 exam result (regardless of who scored it), #2 by #2, etc. etc. So maintaining a high ranking throughout the year can act as a type of insurance policy. Make sense? It's all detailed somewhere deep in the VCAA site if you can be bothered trying to find it

.