All of the kids at my school who had leadership roles in Year 12 also did really well at VCE. Our dux (98.65) was social justice co-captain. The other social justice co-captain and the school co-captains all got ATARs over 90. The social justice co-captain also got top scores from our school for two of her subjects: Psychology and VET Business. Our performing arts captain also got an ATAR over 90.
In other words, students with leadership positions still did really well. Whether it was due to their personalities (and that diligent students are the type of people who apply for leadership positions) or because of the motivation and other benefits of the position, who knows?
Many of the school leaders also won awards (because of the work they did in their positions) which will also look really good on their resume. These included the Caltex All-Rounder Award, Community Service Award, Natalie Vita's Writing Award and Mary Aikenhead Ministry's Social Justice award. Every award given to a Year 12 (with one exception) was given to someone with a school leadership position.
TL;DR- Having a leadership position in Year 12 does not sabotage your VCE or your ATAR, as long as you can find a balance. There are also many benefits to having a leadership position.
I get where you're coming, but for my school there generally wouldn't be more than 3 people getting Atars above 90 and the dux who got above 98 wasn't a school captain, vice school captain or portfolio captain. Maybe he was Class Captain or something like that but not much more.
Also, every year one of our School Captains will get a reasonably high Atar, but not more than 80, as far as I know anyways. However, I do agree that students that have leadership roles would be more likely to get awards which you could put on your resume. I'm not considering trying out for such a high leadership position because this would involve your peers voting for you, which I don't think I'd go well in from past experience.
However, I do want to try out for other leadership roles. We have to sign up to attend an afterschool leadership session (which I'm fine with) and a full day leadership program where we'll miss an entire day of school, which is why I'm reluctant to put my name down for it.
I just want to try out for something like House Captain not a portfolio captain and ussually hardly anyone tries out for this role, so I'd have to make a speech but it would still be likely that I get. But at the same time, I don't want to miss a day of school for this.