...ISO 6400 is not really usable on 5D3, so on 7D2? Not gonna happen... I guess this comes down to the definition of "usable". My shooting style is to avoid high ISOs especially when it's dark....
This is the first I've ever seen this opinion, I find 6400 quite 'usable' on both my 5Dmk3 and 6D. I do agree that one must define 'usable', and if you don't want to see any grain in your photo when you view it at 200% on a monitor, then perhaps you won't find it 'usable'. However, if you want to make nice looking prints, even at rather large sizes, a well exposed file at 6400iso from these cameras should do nicely. As far as avoiding high ISOs when it's dark, well that simply makes no sense at all. I suppose when it gets dark you lower the ISO? Well, no comment.
As for the 7DmkII, or whatever it will be called, I'm not too keen on cropped sensor cameras anymore. My 7D may be the last one I'll own. I think the cropped sensor has 2 places, for cheaper cameras like the Rebel series, and perhaps for a high performance sports/action camera where the longer effective focal length of tele lenses becomes an advantage. For the later, high FPS and good high ISO would be be important, so the 2nd option would be preferable. I should think that 'usable' 3200 iso would be reasonably easy to achieve these days with current technology. The 7D really isn't that bad at 3200, it just loses detail when compared to it's full frame alternatives because of heavy handed NR. They just need to get the high ISO detail retention up, and improve the AF to the accuracy of the 5D3 and it will be attractive to sports and other similarly minded photographers. For portrait/wedding photographers like myself, I feel the cropped sensor cameras are no longer of value.