Only keep extension 2 if you really like maths.. it costs you soooo much time and effort. I think I spent over 50% of the time studying for 4unit. But doing 4unit really helps 3unit, coz you feel 3unit is so easy as the questions are much more straightforward than 4unit. 4unit does scale well, even 50% raw is around a low band 5 which is around 97atar equivalent. Since you got 62% in 4unit, that's not too bad.. maybe keep it until your half yearlies. if you can sustain around that mark, your ATAR will be very nice. If 4unit is stealing a hell lot of time from your other subjects, that you feel you have almost no time to study for them, and your marks are lowering, then drop 4unit.