Thanks for your insights. Considering that my work exacts quite a toll on my bodies, I'm not sure I want to spend $7000 for another one.
If I did, the 1DX would be the obvious choice. So, I'm considering (in no particular order) the 1D4, 5D3, 1DS3 (likely used) and 7d.
I'm a little hesitant about getting the 5D3. I have a 5D2 and find it a bit "fragile" for my tastes, or at least the situations I find myself in. I'm just not sure how well-sealed it is, an important consideration in that I'm around spray and splashes much of the time.
I'm told the 7d is actually more weather-resistant, with better autofocusing as well. And, since I'm usually working at 200 or 400 ISO, noise shouldn't be a problem.
The 1DS has been my favorite for several years, I've had them in different incarnations. They are great performers, reliable and rugged (though I have "trashed" a couple of them, too), but I suspect the AF is somewhat less quick (or as accurate) as some of the other choices. It works very well with the 300/2.8, but it works slower when a converter is put on (somewhat noticable with the 1.4x II and definitely so with the 2x)
The 1D4 could be a good choice. I've used one for a couple shoots with good results. I'm not sure how much more accuracy in focusing I gain over what I already have, and whether it comes close to the 1DX or 5D3.
I'm wondering as well what impact my choice of lenses bear upon the results. Since I use the 300/2.8 more than anything else, it's likely to be my most important consideration.
A 70-200/2.8 is my other workhorse. I just got the II version, but still suspect it doesn't focus as fast as my 300 does (more elements to push around).
I have a 400/5.6 that I'll use when I have plenty of light and may want to travel a bit more lightly. But, I think the AF works more slowly than with other lenses, as my percentage of sharp shots drops at the same exposure settings.
I'm considering replacing it with another 400 (either a 2.8 or 4 DO) or maybe a 500, though almost certainly one of the older versions.
I should also mention I have 1.4x and 2x converters, both version II, that I use, though would likely soon replace the 2x with a III.
Based on your collected experiences, what I wish to know is what configurations work best, in terms of fast and accurate autofocusing (and maybe which ones I should avoid). While I already have some pretty good ideas, your recommendations would be most appreciated.