You need 51% out of each exam weighting for Adv to Scale the same as the standard otherwise you fall below the cohort. Standard and Adv share 2 common modules- with the right teacher and attitude you can perform just as well if not better. If you don't meet that 51% on EVERY EXAM you are risking your ATAR. There's no point in doing Adv English if you're not doing extension as well (just my experience) As for content-heavy subjects, all of the humanities are extremely difficult but SAC has the most content with a major work in yr12. The scale for SAC this year is higher than SOR and Modern too.
The idea of having a common module is that a certain mark in standard can be matched to a mark in advanced based on the whole state percentiles in the common module, and so they can be converted to a fair scaled mark. There’s no “cutoff”, it’s just pure statistics, and marks below 50% scale just like marks above 50% - as an aside, a raw mark of 50 puts you in the bottom 7% of the English Advanced cohort, and an aligned mark of 50 puts you in the bottom 1% of the English Advanced cohort, so if you’re aiming for a high ATAR and getting that low, then you probably have other problems. There’s no such thing as being labelled as “falling behind” the cohort, even below 50% you’re still within the bottom 43% of standard students, so labelling that as “falling behind” would be ridiculous. Put simply, there is no scaling/cutoff conspiracy that should change your decision between standard and advanced.
The big difference is teaching quality, class quality, and effort. Most schools allocate the best teachers to advanced, have advanced classes full of motivated students who will push you up and require significantly more homework (and work in class), which makes a really big difference to your performance. As there’s no effective scaling differences, if you’re a dedicated student who tries hard anyway with an amazing teacher then you can absolutely overcome this (which is really impressive!) - but judging by the fact that just 1% of standard students get a band 6, very few students do. Personally, when I moved up from standard to advanced, I was immediately motivated by the nature of the class to try much harder, which leads to a much higher ATAR, irrespective of any difficulty or scaling differences.
I have nothing against SAC - in line with what I was saying earlier, if its content heavy then that’s a good thing, and if you enjoy it then there’s nothing wrong with doing it!