There are essentially no limits to a student's performance potential, meaning that it is definitely possible to achieve a 98+ ATAR despite C grades in year 11. Of course, while anything is possible, it is important to also be realistic and reasonable. A 98 ATAR is very high and maximising your chances of achieving an ATAR this high (or higher) would ideally involve your performance in all of your subjects being in the band 6/E4 range. It is therefore important, should this be your ATAR goal, that you do your utmost to improve your performance across your subjects, and doing so as soon as possible (for instance, you may need to seek clarification regarding areas of improvement), because the earlier you can optimise your study strategy, the better your position will be as you start year 12.
This brings us to whether you should take Mathematics Extension 2. One could suggest that, given your current situation in Mathematics Extension 1, you should probably not take Mathematics Extension 2 and instead focus your efforts on Mathematics Advanced and Mathematics Extension 1. It is important to ensure that you make an informed decision in this regard, especially given the difficulty of this subject and the fact that sustained commitment will be needed to ensure favourable performance. It would therefore be a good idea to discuss whether you should take this subject with your teacher as they are the person who best understands your strengths and weaknesses and would as a result be able to provide you with direct advice in that regard.
I hope this helps!
