I was about to type "20 747-8Fs - isn't that a bit excessive?", when I thought - hey, this is Emirates! That said, given that they only have a handful of wetleased 744Fs, it does seem quite a jump.
Whatever about the freighters, I'd think an announcement like this would be designed to put the frighteners under Airbus.
If they were to get 747-8Fs, wouldn't they get pax models as well; I don't know what has been proposed about the 747-8 cockpit, but I'd assume that it would have a significant amount of commonality with the 777, which would make conversion between the types relatively easy.
Although EK is apparently waiting for the A370 next month, given its overall annoyance at Airbus, is there any tiny chance that EK would choose the A370over the 787-10? Doubtful, I'd say.