I did History Extension this year. I will say this; it is not an easy course. Much harder than Modern and requires a more detailed analysis of sources and historians!!! Some find it really tedious just analysing historians, others do not. You are guaranteed to have drop outs though! My class only has 5 people now. But tbh I absolutely loved the course though, very interesting but you must do most of the work yourself. It involves reading and that person who said the exams aren't hard, well he's wrong. They are harder than Modern in terms of analysis. Section II is easier than 1, However, Section II is not very hard if you can analyse well because every question is basically historians' differing interpretations, how context shapes debate or how it's changed overtime. And with Section I, it can be similar questions but not always and you have to look through a source, use quite a few historians and bring the debate of postmodernism/historical objectivity.
All this being said, If you love history; do it. Scales well and you learn really interesting things, how to analyse even more effectively than Modern and thought-provoking topics most people would be unaware of. Even if you aren't that good at Modern, it doesn't matter. One girl who failed her Modern trials got 75% in the history ext trial. In History extension (specifically for the case study) you really don't need a lot of knowledge as to what was actually happening, like for Winston Churchill we studied his failure in gallipoli but it's all about Analysis of historians. Their context, how it affected their work etc.