Ross Huggard said at a lecture that I went to, the essays that score 10 are generally around the 800-1000 word mark.
Putting it out there, I heard once that Ross Huggard is a bit of an idiot in general.
I honestly believe even if you are the most succint writer in the world, fitting enough content into 800 words for a 10/10 is hard. 1000 or even 900 words (particularly the former) sounds a bit more plausible.
The thing with very long essays is that they tend to ramble, by the end at least. And often evidence and concepts are drawn into the discussion which are not wholly relevant. Thus if you write a long essay it is easy to fall into these traps and be marked down for it.
I think the thing about very long essays is that unless you feel reasonably comfortable about English, you're not going to write them anyway. At least in my experience, people who do the whole 1200+ words gig are generally fairly confident that what they're saying is relevant.
With that said, I think we should just conclude with the usual maxim - quality over quantity. With that said, if you've got quality down, I don't see why length would be a disadvantage to you as long as you felt it was necessary.