There’s nothing wrong with getting ahead as I did the same with all my subjects (Economics 3/4 in year 10, methods and Spesh 3/4 this year, light reading for Revs last year- completed it this year) accept chemistry so far (probably why I’m doing badly at it LOL) and English where I do pretty much the same stuff next year as I did this year. Just make sure you actually enjoy the subjects that you are actually studying ahead for, as it will keep you motivated to do so. Like I read through the economics ¾ material when I was in year 10, not particularly because I wanted to ‘get ahead’, but because I was really interested in it and wasn’t studying anything at school related to it. I don’t really think I could have done the same for Chemistry to be honest, as it’s quite complex and a bit dry at times. For Spesh I had a tutor to teach me the concepts, and for methods, I studied on my own-watch videos and do the Essentials Textbook problems. For Revs I just read relevant material during my term holidays last year. I would recommend against cramming too much of the course in a short period of time, if you’re ambitious maybe just try to cover unit 3 during holidays and unit 4 during the mid-year holidays. 
This holidays I might do more random maths problems, revise unit 4 economics on policies, revise stoichiometry and relevant ½ parts for Chemistry, read books for English and revise topics such as complex regions, vector proofs, kinematic and dynamics-which I'm not really clear on. 
Good luck  
