I completed Advanced, Extension 1 and Extension 2, was extremely passionate about English and still ended up being one of the only people in my cohort to receive a band 6 in Advanced English. I also found Extension 1 and 2 were more demanding of my time because there's a higher degree of sophistication needed for the critical responses and creative pieces than Advanced. I guess it also helped that the texts I analysed in Extension 1 were in fact extensions of concepts I was learning in Advanced anyway. Whilst it may be true that some Extension students may not perform as well as Advanced students due to workload stress or an inability to get to the crux of an argument because of their 'overwhelming passion' which causes them to go on unrelated tangents to the question, I would like to add that passion for a subject is generally a good indicator of how strongly you will perform in it.
This was not written to diss your point or shut you down, I was just offering another perspective from someone who is immensely passionate about English and could still do well in Advanced.
I'd like to add to this
I did Adv and Ext, and defo enjoyed Extension a lot more. It was more demanding, as you said (especially because of the level of independent work required), but the texts were much more interesting than those in advanced and I genuinely had more fun writing my essays and creatives for extension.
I ranked first internally for both advanced and extension (with B6 and E4 in externals). In externals, everyone in my extension class was literally one mark behind the other; this is something that was consistent throughout year 12. And all of us did better in extension than in advanced when it came to our external marks. I can't speak for my classmates, but I can speak for myself. The passion for English was definitely there, and it was the subject I spent the most time on, always refining essays, doing more creatives, practising short answers, etc. It was never a matter of going off on a tangent in advanced, as someone mentioned previously, because essays were always direct and well-synthesised.
I think, in the end, it comes down to the way in which extension students end up wiring their brains to tackle essay questions: we immediately switch on into an intensely analytical mindset, creating our own theses and establishing multiple links within and between texts/contexts/composers. In an advanced essay, this can often come across as a bit much (bear in mind that extension markers and advanced markers are different sets of people with different expectations). On numerous occasions, my teacher (who taught me for both advanced and extension) told me to dial back my analysis, or my links, or a contextual concept I had researched and included in my essay, simply because it was far too above and beyond for an advanced essay.
I walked out of Adv paper 1 and paper 2 much more confident that I'd smashed them than extension 1, yet extension ended up being so much higher (even when it came down to raw marks). In the end, I think it comes down to the nuanced relationship between the student's approach to any essay question or creative prompt, and the marker's expectations.