Unfortunately, the way that the vce system is set up stuff like cheating or teacher's favouring certain students does impact everyone's final study scores.
Sac scores are always important but especially in English where 50% of your study score comes from those marks. Anecdotall,y I have seen students who have scored extremely high exam scores (equivalent to high 40s/50) but end up with mid-40s due to their sacs. On the other hand, people who have done really well on sacs have boosted scores before. E.g. A+ sacs with an A on the exam yielding 40+ scores.
An exam score of 48-50/60 is actually quite good and usually an A+. On the face of it 80%/85% sac scores are not bad in English but it would really depend on what the cohort average is an how strong your cohort is. I think getting low 40s would still be possible with the score you have mentioned.
For your final question, it would depend on how everyone in the cohort does on the exam but it is a possibility.