Firstly, History Ext is an EXTREMELY enjoyable and rewarding subject provided you want to learn more about historical methodology and practice rather than history itself haha.
Whilst i personally enjoy it a great deal, i have to say that the HSC exam is one of the hardest i will ever have to sit. 2 cohesive and comprehensive essays that not only answer the question but also integrate your thorough knowledge of MULTIPLE historians and make extensive and relevant use of (a sometimes very sophisticated) source is a fairly challenging ask. That is not to say that high range marks cannot be achieved, it does however require a fair bit of practice...a face evident to me in my trial exam where I only got mid-B range essay marks following literally NO practice essays. My mark placed me at the bottom of my cohort =(
But anyway, it is also important to consider what you want to do in the future and what ATAR you aim to achieve. This subject, honestly speaking, scales HORRIBLY!! A fact that makes me very very sad because to me it is one of the hardest HSC subjects the BOS offers. Maths X1 on the other hand scales fantastically in comparison, so if you are doing well in it i recommend you persevere with it. That is not to say dont pick up Hist Ext, because if you can do well in this subject you will be rewarded immensely as well...you just need to get a lot higher in it to unlock these rewards =/. For example, a HSC mark of 92 in MX1 was scaled by UAC to 89.6 whereas a mark of 94 in Hist Ext was scaled by UAC to 84.6!
It is also important that you have a good idea for your project and keep up to date with your research and logs throughout the year!! This aspect of the subject is EXTREMELY fun and you get to learn about an aspect of historical debate that interest you a lot more =)
Overall, i do encourage you to give the subject a go...it can be a big hit and miss (if you dont like the content or find it too hard or cant formulate an idea for your project) but it can also be excellent if everything falls into stride for you
I know for me, this subject was EXCELLENT till the trials, after which things kinda went downhill =/ but practice makes perfect and that is something that I (and you will) need to do