From what I can see they've done a lot. I think standard cores of any branch would struggle with so many players. They've worked on prioritizing activities as much as possible there's probably little more than can be done. Aside from splitting the server into nodes, in the same way retail does. But, that'd be a huge dev undertaking and not something you experiment with 6000+ players with
Likely if they expected to get this many players, they would have considered a solution similar to this.
From my point of view, during busy periods it is undesirable. But, it's not as terrible as people make out. probably for PvP it is though.
Casual and proud.