Also, I was wondering about how important the spread of SAC scores is. At the end of the year if #1 and #2 are separated by 1 mark, would it be different if the separation was say 10 marks? I'm a little unsure about this because I have heard teachers say on many occasions that 'SAC's are just a ranking.'
no, the separation between a 1 mark or 10 mark difference in your SAC performance doesnt directly affect your GA grading.
however, SAC performance [especially the overall, averaged performance] is a good prediction for exam performance at the end of the year, so the exam performance of two people very close together would be similar, if there were a huge gap between two people then a similar gap should be expected in the exam results.
so to answer your question, no, the degree of separation does not impact much [if at all], but it can [and does] reflect exam performance.
bottom line: do as well as you can =]
and:
In subjects like the Sciences, Methods, Spec there are 100 marks for SAC's. Does this mean that the whole year's SAC's go towards one single ranking and there is no separate Unit 3 ranking and Unit 4 ranking? (unlike English for example)
that is correct.