I thought I'd offer some insight as someone who quit debating in year 12 - I did debating from year 7 until 11 and quit in year 12 as I was worried that the time commitment would sabotage my VCE performance. In hindsight, I really regret it. From year 7 till 11 I've never intentionally put effort into English, or any humanities subjects, but I was still passing with decent grades.
I found that the skills you get from debating - which isn't just limited to public speaking but also things such as thinking on the spot, writing persuasive arguments, addressing key points and sticking to the contention really helps with English and humanities.
Debating has very similar criteria to these essay-based subjects - establish a contention, develop key arguments, introduce evidence to enhance your points, and ensure that you are getting your message across. It really helped with my essay writing even though prior to year 12 I never consciously put effort into trying to improve my writing skills.
So yeah - debating definitely helps - not just with English but across all sorts of essay-based subjects. If I could rewind the clock I'd definitely stop myself from quitting at the start of last year. But that's just my experience with debating.