John, I saw your post in another thread about the 800 even being too short for you many times. I did a CPS loan of the 800 a couple of years ago and found that to be the case, too, even on a 7D. I thought the 800 would solve all of my problems by getting me close, but that's when I realized I really need a 2000mm to get those shots! And as such, that's when I decided to change my approach (literally) to wildlife and work on getting closer to them for the shot. The 300mm (with or without extenders) turned out to be the perfect lens for that purpose because of it's small size & weight, and the IS in the Mk II is so good, I have stopped using a monopod, let alone a tripod for most things. Some other members on the forum and Canon Explorer of Light Art Morris (see his related blog posts here) is also finding this lens + extenders combination to be well suited for his work, so I'm not alone in my opinion. Obviously it's not for everyone, but if you can live without 601mm+ or the convenience of the 200-400 1.4x, it's a nice set up for everything from sports to wildlife, especially with the 1D X and 5DIII.
You know you want one - just do it!
The only question is which one. To me there are only two choices, either the 800mm or 600mm, I suppose you could include the 500mm for mobility.
Many people go on about lenses being too long - this is the opposite of my experience, in fact on only 2 occasions has my 800mm proved too long normally it's too short!
I most definitely would go for the 300 f/2.8 IS II because it's not too heavy of a monster and you have the option of extending its range using tele converters. Such a lens would have to get a lot of use from me, and I usually don't have a need for 400mm plus.
I have the 300 F2.8 but, unfortunately, it get far less use than my 800 F5.6 L. The 300 is a great lens and very portable, plus it works well with extenders. The 800 is just better for my uses.
The 300 F2.8 (Mk1 or Mk2) is a superb lens and really does well with extenders. I am more than happy with the performance of my 300 F2.8 IS Mk1 with my 2 x Mk3 extender. A few weeks ago I took 2400+ shots at an Air Display (sorry I got a bit carried away!) - my 300 F2.8 both with and without the 2 x extender, on my 1DX, did not miss focus on a single shot! IQ was excellent also.
However for my main pastime (small birds) the 800 F5.6 gives me better results and faster AF. As to getting closer I am normally shooting between 20 and 40 feet. I have to admit though the 300mm wins hands down if I need mobility!
I am interested in your comment on the Mk2 IS. My 300 has 2 stop IS and the 800 4 stop IS however I nearly always have it turned off as it improves the AF performance and, with the 1DX, shutter speed is rarely an issue. In fact the only time I have used my IS (on any lens) this year was once when hand holding my 800mm and then it was only to steady the image in the viewfinder! Shutter speed was 1/2000 sec so IS wouldn't help the image but it did help me get the focus point where I wanted!