hey. it really depends on the weighting of the trials upon your school mark. You need to find this out to know how much can be changed into your favour. For me, Maths ext 1 is 40%, other subs are 35%, english 2 unit is 25%
my school is different to yours so you need to find this out
for hsc - yes you can do well in these to get a 55 atar at least. but when you mean 'really' well, how well do you mean? If you've been getting idk like 50% for your previous assessments on average, maybe if you wanted to get a 65-70 in your final school report, you might need a 80 in trials for example, if your trials are weighted 50%. but this really depends on the weighting of your trials. If it's only like 35% like for me, you might need to get 80-90s to offset your marks you want to improve to just get a 60-65.
your hsc exams are 50% of your final mark for the course. but that doesn't mean to slack off in trials, your trials are like mock exams for the hsc, so it's not like you are wasting your time by doing trials. treat your trials as a feedback on what to do better on in your hsc exam for your subs. good luck