Plotting a graph of ΔATAR/study score vs ATAR gives an interesting curve: students whose ATARs are around 50 have the most to gain from an additional study score point. Above about 90, the incremental ATAR gain from a single extra study point is probably below the margin of error given the way in which ATARs are calculated.
Tip for students: check the entry requirements for your course and make sure you meet those first. If your requires, for example, a particular score in the UMAT or in English, make sure you get that score. If your course requires a particular ATAR, make sure you get that, too. Remember that these scores are just entry requirements for undergraduate courses; not indicators of self-worth.