USyd is your dream destination right? I believe you should start by weighing the positive aspects and the negative aspects of completing the MOOC.
If your dream course is at USyd, then perhaps completing the MOOC would be a good idea, especially if you meet the ATAR requirement for your chosen degree. It seems that all registration deadlines precede the release date of students' ATAR's so it doesn't seem like you will be able to complete the MOOC after you receive your ATAR. If you choose not to complete the MOOC but end up meeting the ATAR requirement for your degree, then I can imagine a negative feeling which I would not want to experience, especially if this were my dream course.
Although you will be allocating time and spending money to complete this course, I believe that wasting them would probably be better than deciding to not attempt the course at all, especially in the event where you meet the ATAR requirement for your degree.
I hope this helps!