It irritates the snot out of me that this fix has been available for the 5D3 for more than a month now, but still isn't published for the higher end model. Meanwhile Canon seems to be focused on new lens sales more than fixing performance issues in their flagship photo DSLR.
In fairness, they said 'late May' for the 1D X update, and the month isn't (quite) over yet. The 1D X got f/8 AF 6 months before the 5DIII, and the 5DIII apparently isn't getting the AI Servo AF point illumination fix, ever.
I suspect what happened in this case is that Canon had pre-announced the April 5DIII update for f/8 AF and clean HDMI out, and it made sense to roll the AF Assist update into that release.
Are people really going to use an external 1.4 extender IN ADDITION to the built in 1.4 extender?
I'm sure some people will, yes. I wonder about the IQ, though. Worth noting that there are two ways to get to 800mm f/8 with the 200-400 lens - built in 1.4x and a 1.4x behind, or disengage the built in 1.4x and put a 2x on the back. I presume the latter does not require a firmware update to AF on 1-series bodies. I'm sure TDP will test both combinations, but if I had to guess, I'd bet that the 2x without the built in 1.4x would deliver better IQ than two 1.4x TCs in the optical path.