Hey Jack, I was in a similar situation to you. I came to Aus when I was only 4, and I mainly spoke a dialect in China. It was a real struggle. I had the same issue and thought it was completely unfair. I had a lot of arguments with my parents and the subject emotionally strained. Depending on your current situation, I would say that it really depends on how good you are now. Although I came when I was really young like you, my parents always spoke Chinese mandarin/dialect to me, so I was somewhat fluent in Chinese – albeit a lot of work was still needed to be done for GC and DS.
The real game changer at the end of the day for me was having an amazing teacher pushing me to get a respectable score. She helped me every step of the way to get there. My class mates were all better than me but through diligence and hard work, I got a decent score for the disadvantage of coming early.
Another thing that you should consider though is that Chinese does not really favour people who come super late either. Those that come late do not necessarily do better. Some of my friends at school got low 30's even though they came after having an education in China. The exam and SACs are all about 技巧 and not your actual skill. Keep in mind you also have an advantage for translation!
Hope this helps, if you need more help, pm!