Can we expect any kind of performance impact from the AABB bounding?
I'd say it might improve things a bit. You could cache bounding boxes for specific states and return them more easily, I suppose.
EDIT: I'm going to add to that to say that it does mean creating a lot of junk for the garbage collector to clean up when a block needs very many bounds changes, since a new AxisAlignedBB has to be created for absolutely any change during rendering or whatever. But Mojang has already been heading in this direction of lots of spillage of this sort, so it's status quo at this point.
Searge and Dinnerbone were tweeting about how they worked so much, so I was guessing this snapshot would have been awesome anyway, yet they were also talking about how their work could break the game....
Yeah Searge said something about reducing the number of times a chunk potentially needs to be generated. I don't know what specifically was changed there yet.