Its not really a good subject. The scaling is not great tbh and a lot of people complain about the workload. It honestly depends on ur school because the exams can be really different depending where you go. And as a single unit idk if it will affect ur atar very much. Also the final report is INTERNALLY MARKED - which means it only affects your ranking. The hsc exam is this kinda like a biology skills assessment (reliability, accuracy, validity, assessing sources etc.)
Personally I enjoyed it. My school only had two internal exams plus the report. The report only took me a week to write and I got 38/40. My experiment was on plant growth so it didnt take much of my time apart from setting it up. See if your school has the report due before or after trials. Mine was after so it was chill.
If you do well in depth studies in other science subjects I would recommend science extension. If you come up with an idea early its really not that much work, and the actual content is really easy.