Hit 85 on the Death Knight finally yesterday and have already gotten going on the mage, who is almost to 81. The Death Knight was very easy to level, and ended up being my second fastest leveling time despite leveling mining all the way to max.
I stockpiled what I gathered and will come back to leveling blacksmithing later on.
I did try a handful of dungeons on the death knight just to see if my previous conclusions on xp per hour had changed. In both cases, the dungeon resulted in a significant drop in xp per hour.
When I start the rogue for my final toon, I may try doing some battlegrounds on the rogue, just to see what the difference is there.
In the case of the DK, during Deepholm and Uldum, running instances dropped my average xp per hour to right around 1 million, from 1.5 million which is what I was easily hitting while questing. And to drop the average to that, the total xp per hour while running dungeons must actually be much lower than 1 million.