I went shooting yesterday with the 1DX and the 300mm f/2.8L IS + 2XII and I felt it was too short for most birds and I couldn't get enough reach to the Osprey nest. I was also not too pleased with the decreased sharpness, and slower AF speed on the 2XII on the 300mm version I. I know that the 300mm version II and 2xIII are better, but it still must be a compromise.
The 1Dx is pretty heavy and now I was using the 300mm with 2X, and I used to use the MKIV 300mm + 1.4X. I definitely felt the difference.
Ok, so the new super-telephotos got lighter and our camera got heavier. In case anyone didn't notice the 2XIII also gained about 3 ounces.
I went from the 7D (1.6X), to the MKIV (1.3X), and now the 1DX (FF).
"It seems the more I spend, the further back I get, financially and figuratively!"
John, If I were you I wouldn't get the MKIV, because you've already experienced the 1DX, and the MKIV is a slight step backwards in AF speed. I would rather see you put the MKIV money into longer glass. If you don't need f/2.8 for indoor events or night games, then you should also pass on the 400mm f/2.8L IS II.
So, it comes down to the 500mm or 600mm. Most people say to go for the longer lens for birds, and now that is especially true with the FF 1DX. With the MKIV, I think you could make an argument for the 500mm with and without the 1.4X, but then 700mm will be the longest reach without giving up AF, as compared to the 600mm +1.4X @840mm. If 700mm is long enough, then 500mm would be a nice compromise, considering cost, weight, and flexibility. If are more interested in sports, larger game wildlife, and portability, then the 500mm would be the way to go.
Canon will eventually come out with a new 1.6 Crop Camera with the improved AF and noise capability, then the 500mm would suffice.
I think with these new high ISO, low noise cameras, that the 500mm will become more popular for sports. Keep in mind that the 400mm on the MKIV was 520mm. Everyone needed f/2.8 for for speed to keep the noise down. Now, people will be able to shoot at 25,600 max instead of 12,800 max, because we've gained a whole f-stop over the MKIV. Of course if people choose the 400mm f/2.8L on the 1DX then they will lose the focal range from 520mm to 400mm. Then if people add the 1.4X to the 400mmm to bring the focal range back up to 560mm (which also gets a little long for sports), then they will be giving back that f-stop again, and slowing down their focusing speed with the extender.
I see the 500mm/4L IS II as becoming more popular for field sports with Canons new flagship FF 1DX and even with the 5DIII. Now, for small children, the 600mm will still be pretty good for field sports to get those cute isolated sports portrait shots. As they get older, and faster, then it will be a little too long.
If Canons 70-200mm II and new super-telephotos are any indication of which direction they are headed in, then I think we will be able to look forward to a new sharper 100-400mm, as well as the new 200-400mm +1.4X @ 560mm. The only downside to the 200-400mm @560mm is that it won't be long enough on full frame with smaller birds. I could see the 200-400mm being great for sports and air shows.
Rich