In year 11 i was in a similar position to you (I was failing!!!). So deciding whether to go to ace or not, I eventually chose the latter as i felt that it my time may be better spent on english :)
Here's the thing buddy,
ATAR is JUST A NUMBER. So don't let it define you!
Nevertheless, I ran you predicted marks based on your school ranking (high 300s) and theres a chance you could get an 85. But, take it with a very teenie tiny grain of salt (it may be higher)