I'll have to agree with argonaut here. Regardless of whether you like the assigned modules or not, you're going to have to sit a SAC on them anyway and you're going to have to learn how to minimise silly errors in them if you want to do well in them. You're not going to get any support whatsoever from your school if you learn extra modules on your own, nor will you get much practice come exam time - to save paper, schools tend to just print out the relevant modules when handing exams out to their students. Not to mention, you seriously will forget them come exam time if you start now. The only time you should bother learning a new module is if you're not understanding the topic at all and you haven't done well on the SAC. It's worth it only to save your exam score from serious harm.
I did Geometry and Trigonometry, Graphs and Relations and Business-related Mathematics as my three modules. When you get to Business-related Mathematics, you'll see that a lot of it is new and unless you're an Accounting student (which you're not) you would never have seen it, and your advantage over the regular Further students is lost. You have to spend a lot of time understanding the theory or else you won't be able to do the arithmetic well at all. Furthermore, VCAA loves testing theory knowledge for this module (Question 9 of this year's multiple choice exam had no calculations in it whatsoever - I got it in a few seconds in reading time while others simply had no clue whatsoever; and remember, Question 9 is always the hardest question). I know a few kids learnt Matrices out of frustration, but they felt a little uneasy doing something different in the exam room compared to everyone else.
Yes, a lot of Graphs and Relations and Matrices is covered in Methods, but many Methods students then become complacent and feel they can fill these modules out on the exam without doing the work in the worst-case scenario, which is not true. Each module is equally prone to silly errors and it's just something you'll just have to work on throughout the year. Luckily, for stronger Maths students, Further is not a subject where time is an issue so I can only urge you to take your time and complete everything once, but properly.
Also, you don't need to do any holiday homework for Further, trust me.
Feel free to ask me more questions or if you need any help. I'm making a bit of an effort to hang around here since I know the Further forum tends to get a bit neglected.
