What I found surprising was the standards of maths improved as the difficulty went higher.

Just looked at a few interesting statistics for 2015 Exam Performance (from the UAC website):

General Maths (Scaled scored out of 50 based on HSC Mark):

0-25 7.5%

25-30 17.6%

30-35 24.4%

35-40 24.6%

40-45 20.2%

45+ 5.7%

2 Unit Maths (Scaled scored out of 50 based on HSC mark):

0-25 3.1%

25-30 6.1%

30-35 9.9%

35-40 28.4%

40-45 32.8%

45+ 19.7%

Maths Extension 1 (Scaled scored out of 50 based on HSC mark):

0-25 1.7%

25-30 4.3%

30-35 9.7%

35-40 20.1%

40-45 29.8%

45+ 34.4%

Maths Extension 2 (Scaled scored out of 50 based on HSC mark):

0-25 1.5%

25-30 3.5%

30-35 8.7%

35-40 18.0%

40-45 32.2%

45+ 36.1%

Median marks for each of the respective maths (General, Maths, MX1, MX2) were : Band 4, Band 5, E3 and E3.

So that tells me that the gap between General and the higher maths is quite substantial.

The cut-offs to make for an E3 in MX2 is 70-89 and an E4 in MX2 is 90 or better. (I am not sure how hard that would be to achieve)