Any Canon 400 F2.8 is a great piece of glass if it suits your needs.
I used to have the original EF 400 F2.8 L (a mere lightweight at 6.1 kilos), yes it was an old battered lens but the images were top notch!
The current model is less than 4 kilos and has updated optics - if 400mm is your bag then you will be very happy!
I note you use a 1DX, given it's iso capabilities, I would suggest turning off the IS as auto-focus and tracking will be even faster - if that's possible! Since January I have used IS on one of my lenses only once and then it was only to stabilize the image in the viewfinder, even with my Canon 800 F5.6 I find IS to be more of a hazard than a help most of the time with the 1DX.
This is very interesting. I've never heard that before. Does anyone else find IS interferes with AF? Is this just a 1Dx issue?
I think my statement "I find IS to be more of a hazard than a help most of the time with the 1DX" probably wasn't the best way to put it! Basically what I have found is that all my IS lenses focus a touch quicker and seem to track better with the IS off. This was on a 5Dc, 1D4 and 1DX. The difference that the 1DX makes is that IS is needed far less due to it's ISO capabilities.
For reference I don't have any Mk2 lenses so I don't have the "Mode 3" that has been mentioned. My IS lenses are the 24-105, 70-200 F2.8, 300 F2.8 and 800 F5.6 - the 800 F5.6 shows the most noticeable improvement. The difference is not night and day but it just locks on that bit quicker and seems to hold on to the subject a little better.
I am not knocking IS, far from it, it is just that now it is something I keep in reserve for when it is really needed.