The thing is...Canon's 400 f2.8 LIS and it's subsequant mkII version are a two of Canon's finest lenses todate. They are some of the sharpest and most optically impressive lenses ever made by anyone. Nikon will have a hard time comeing close to Canon's products in this niche. Nikon's 400 f2.8 hasn't been in the same league as Canon's for some time and trucst me when I say that even the mkI LIS version is truely exceptional optic. On my 5DIII i can see very little difference wide open and stopped down and wide open with 1.4x TC...better than a 70-200 f2.8 II LCurrent canon 400mm f2.8 IS II is solid. Will be a challenge for them.
There's no IQ upgrade from the mkI to mkII unlike all the other new white teles, which says a lot about the Canon mkI 400 LIS. Sure the new one is lighter but from an optical point of view, there is little between them.
Even with weight reduction on mrk II, it's still on heavy side for handheld. I can shoot 2-3 photos still subject. I hate it when I have to track the moving subject....it's no fun.
I used mrk II at least 3-4 times a month for indoor swimming and ballet. IQ amazed everytimes