Your rank in a particular subject (in this case Studies of Religion I) is one of two elements that is involved in determining your Assessment Mark (internal mark), the other element used being Examination Marks achieved by you and other students in your cohort. This is known as the moderation process. This process does not determine your Examination Mark (your HSC exam mark), i.e. your internal rank does not determine the mark that you receive for externals. Your Examination Mark is largely dependent on your own performance in your SOR I HSC exam and is not affected by factors such as your rank relative to your cohort or your school rank.
Because of this, unless you are certain that you would be better off preparing for your other subjects next year with the aim of maximising your performance (as you mentioned, SOR I will not count towards your ATAR if it is your lowest performing subject since you are taking 11 other units), you should thoroughly prepare for your exam in this subject. This would also be my personal recommendation because this is the only HSC exam that you have to worry about as of right now, so you definitely have nothing to lose.
I hope this helps!