Well i don't follow that outside the box thinking. Get a 7d when you have a 5dmk3? So with the 7d he'll get twice the DOF, thats not going to help isolating people in sporting situations. Then he'll be demoted to 18mp and lose, what, 2 stops of ISO performance, and get a much weaker AF system, less spiffy LCD etc. If you have a 300mm and a 5dmk3 and you think you would like a longer setup occasionally, I would get the 1.4 tele and if that doesn't get it done, crop. Or just crop now and buy nothing. I get amazing prints from my 13mp original 5d. so with my newer mk3 with 23mp to burn i see that as a built-in 2x ext when neeed This setup isn't to far removed from mine and I've got no complaints. I run the mk3 with a 300is2.8 and sometimes slap a 1.4 ext on it. Unless of course you really need 23mp files for some reason, or want to purchase, learn, and haul around another camera body, with a back up battery.
I also have a 300 f/4 and a 1.4x extender.
The change in AF speed is quite noticeable once the 1.4x extender is installed. You also go from f4 to f5.6. I am not an experienced wildlife photographer, but I view the increase in AF speed as an issue in a dynamic environment. The AF speed penalty is fine for slow moving/static subjects.
I do not have a 300 f2.8, nor do I know if the AF speed on the f2.8 lens is any different from the f4 lens.
The 300 f2.8 becomes f4 with the 1.4x, which is still better than f5.6 unless the lighting is good or unless you can crank the ISO.
I am not a DoF expert, but sometimes, the smaller sensor of the 7D would be an advantage in that respect.
Cropping will only take you so far.
The 7D still puts more pixels on the image to start with than a full frame 5 series or 1 series. So, if the image is good to begin with, the crop of a 7D image turns out bigger, if not better. Maybe not the most correct technical explanation, but the pixel density on the 7D is greater than the full frame equivalent, so the 7D shot has more pixels than a FF crop.
Yes, the 7D has some ISO limitations in comparison to the 5DII or 5DIII. But, outdoors in good lighting, I believe those limitations go away. I have many very nice outdoor images taken with a 7D and they look damn good to me straight out of the camera. They would look better if I printed them and lost the ability to pixel peep on a monitor. Better still if I did some basic post process work. In fact, if i print them on my PS (postscript) 4 color laser printer on 32lb laser paper, many of those 7D prints at 8.5 x 11 still look better than pretty much anything I ever did with a film camera printed at that size.
The images from the 1DIV are better than the 7D, but that camera costs a whole lot more than a 7D. Even used.
Given the OP's description, I still think going with a 7D is a better choice than the 1.4x extender. The word "occasionally" is mentioned, so I am *assuming* that the intent is to install a 1.4x extender when needed. This could also mean that the 300mm f4 lens is removed from the 5dIII and then could be installed on the 7D.... or the OP could leave the 300 on a 7D and then stick a 70-200 on the 5DIII.
There are many ways to approach the problem. In my opinion, the AF speed hit from the 1.4x extender is a big deal.
If the OP does not like the 7D idea either, then I would say go and get a 400 mm lens.
I would like to own one of those 400mm f2.8 lenses, but they are pretty darn expensive.