I think that they're always working on the engine and performance, and lawd, the game needs that and more. I'd expect the expansion will contain plenty of performance work, which may or may not be countered completely by new content and features.
A big part of the problem is that ANet cater to huge zerg events and metas, because people like those (which is great and all) but those are the places that the game becomes a ridiculous migraine slide show. You'd think that performance might be higher up the list of priorities given their fervor for such events and how, as good as they sometimes are, show the game in the worst light possible. Which makes me think we're pretty much at peak performance for what this game can handle without substantial rebuild or something new.
And that degree of effort (read: cost) is _almost_ certainly reserved for sequels.