I think to extend yourself from an 8 to a 9 conciseness may play a role, but only in the manner that it would allow you to provide additional analysis/exploration of ideas. I think that the insightfulness of your ideas and the accuracy in the way you present them is most important from moving up from an 8 to a 9, and even from a 9 to a 10.
I asked my english teacher (who is a VCAA assessor) last year how he differentiated a 9 from a 10 last year, and his answer was simply that the essays he marked a 10 left no doubt in his mind. That is, the logic of the ideas and the supporting evidence from the text in the essay is so well selected and described that there is no room for questioning its feasibility. With a 9, the ideas explored may be the same as in a 10 essay, but perhaps not as well communicated.
I hope this helps!