G'day
Music perf is an awesome subject. Can't really say if its hard or easy, because firstly, that heavily depends upon your attitude towards the subject as well as your musical ability. Secondly, the way music perf is approached by each school varies quite a bit. At my school, written assignments weren't really a big deal. We mostly concentrated on the SACs - as there were multiple parts to them including aural, theory, tech work etc, as well as an analysis of the tech work we played.
I would encourage you to take it on - I did quite an academic yr 12 program, and music certainly took off the pressure as it was such a different subject to do. The value of having a 'different' subject in yr 12 cannot be underestimated - I think this reason alone outweighs the 'risk' of doing a subjective, creative subject. At the end of it all anyway, the practical examiners are pretty fair - as long as you are convincing with your performance, you'll do fine.
Don't let a fear of written assignments deter you. Its music performance - that is what really matters.
Hope that helps!
Good luck and feel free to pm me with any queries!