Looking forward to both of your findings.
It is an exciting time in Photography in terms of tools advancement. Now I just need to step up my artistic skill sets.
Of particular interest to me is focus and tracking of moving subjects. If the new generation make it significantly easier to lock on to the eyes of a raptor in flight, then it may be replacing my d300s. To be truthful, I'm not too interested in having 30+ megapixel images. I would probably drop the images down to more workable sizes from the start unless it really makes a difference when moderately cropping. I'm hoping that I don't get spoiled, and continue to "just get closer" and create stronger images. But it will be nice to have the option should I have no other choice.
My original hope was to improve on the low-light capture capabilities of my d700, but there is little else that the d800 offers that I find motivates me to retire it. But I am still hopeful that you will find some nice real-world advantages that haven't even been discussed yet.
Oh, and I'm REALLY interested in seeing how the TC-20e iii handles with my 70-200VRii when all that extra resolving power is in play! If it's really good, it just may replace my Sigma 150-500 OS as my hand-holdable bird in flight setup of choice. (I get some stunners with the Sigma, but seldom take it beyond 400mm, so I would not be missing much of that last 100mm...)
You realize of course that the d800 might just force me to re-evaluate every lens I have from my 50 and 85mm 1.4 lenses to my 300mm 2.8! I'm already expecting my Sigma to not make the cut, but I may be very surprised...