Exam-wise, if you're scoring above a 60/75, it's good (i.e. 40+. sensei's rule of thumb is take 20 off your exam score). Don't trust exam scores from last year - it was apparently the easiest Japanese exam EVER. A girl from my school last year who got 49 raw got 72/75, with a 14/15 in her essay. -shudders- kowai.
Kanji-wise, unless it's going to reduce the amount of ji in your essays, it's probably unnecessary to learn them (unless you're like me and just do anyway). I advise learning the one for はたらきます(働きます)
For grammar, learn all the VCAA specified ones and then passive, causative and causative-passive. It covers everything you need to know and shows some depth. And use a variety of grammar in essays etc.
With top scorers, I'm probably the least qualified to speak about ranking and such. It's so competitive in my class it's crazy ==lll But take examples of all the 40+ scorers with Japanese names - only a few get those really high scores. It's about hard work, like it is in every subject over a certain point.