I think studying the whole course before the year even starts is a bit too far, although I think it's more about finding what works best for you. I started a little early in the summer holidays for Methods and Specialist, I didn't do the whole course, but did what I was comfortable with. Rather than saying "I need to get this done by this date", I just worked through at a pace that allowed me to deeply understand the content. What I'm getting at here is that if you're going to study ahead, then you want to be learning the content properly. I think this may be one of the reasons some people who do everything too early don't end up doing as well, they think they've covered everything that they need to do, and as a result slack off in class, which in some cases means they miss key points or concepts that they didn't realise that they'd missed.
You don't want to burn yourself out early though, you need to have a big enough rest that allows you to take on the year properly.
However, if you are motivated, feel like working ahead because you enjoy it then go ahead. I just don't think it's necessary if you feel that is mandatory, and feel forced to do it. I'd recommend working ahead in the summer holidays, by opening your book when you feel like it, and probably doing the first couple of chapters.
Totally agree with this.^
The second area where I think that causes some trouble is (it's partly already mentioned) is that you need to be constantly revising throughout the year. Some people I've seen finish the course real early and then slack off and not revising. As a result all their hard work at the start of the year is lost, and as the content now seems dull to them, they have trouble getting back into it and re-learning it again and as a result their results suffer.
It really depends on the person but if I commenced the year whilst completing the course what would I do for the whole year?
I'd most likely just slack off, waste time, and forget everything I've learnt a year ago.
Now this is what seems to happen, but what you can do, something that I did while working through the end of the course material for Methods, is to go around and help others in the class out. If you can explain a question/concept well enough for someone else to be able to understand how, what and
why, then you know that you've understood the material properly, it also keeps most of it fresh in your mind (this is more thinking if you were studying ahead to finish a term or so early, not a whole damn year early). This also helps prepare you if you want to go on to do some tutoring next year

Overall I think that studying ahead
can be a good think, depending on how you learn. If you can be a great independent learner and make sure you do miss anything, then sure, go for it. But if you're going to struggle to learn on your own then it's probably not the best idea. Just don't go into overkill, completing the course before you even start the year isn't always such a great idea. As said above by others, I guess it depends whether you're dedicated enough to keep going over what you've learnt, to make sure it all sticks.
I probably should also bring up not starting practice exams too early, again you don't want to burn yourself out and you want to have your experiences and what you've learnt from prac exams fresh in your mind near exams (not saying start too late, more just don't run yourself into the ground too early). As with this, I should make the point that it should be quality, not quantity. Some people start doing prac exams way early, and don't learn anything from them, or forget what they learn from then since they've started them way back in January. (You can also run the risk of running out of exams, and end up doing the same ones over again, where you lose the element of surprise).
(...started summarising above before tangenting off... soo..) Study ahead if you can, just don't study
too far ahead, don't burn yourself out early for no reason (again this will vary person to person), you just have to find what works best for you.