Well, one of my pals has a Nikon D810 and the IQ is absolutely excellent, crops very well if necessary, but again although the frame rate has improved over the D800, it's still of limited use and he relies mostly on his D4s for wildlife.
We have just returned from the Pantanal in Brazil shooting jaguars, he was using the D4s with the Nikon 80-400, and getting excellent results, it really is a great lens and I hope the Canon 100-400 replacement, which has been confirmed in principle by Canon and hopefully is a turn of the year job, will be as good.
I was using one 1 Dx with a 70-200 mk2 2.8 with a 1.4 extender on for most of the time, and when I needed more reach the other 1Dx with a 300 mk2 2.8 and another 1.4 extender, and I took a 2x extender when I needed 600mm, so I also had the opportunity to drop to 2.8 with both lenses very early morning and late evening, however had the new 100-400 been available the weight advantage would have been significant I believe.
Just what suits your own type of photography!!....and a 7D mk2 would have been great...again weight saving!!
Edit...somehow my last para went missing....
should have added....I don't think there is a substitute for FF and a good prime/zoom, but the price and weight advantages of a 7D2 with a quality zoom will be a very marketable combo, lets hope the result is worth the wait!!