An important thing to remember about dof, if your subject is the same size in the frame the dof is the same regardless of lens focal length. The perspective is different, but the dof is constant. http://luminous-landscape.com/tutorials/dof2.shtml
Now if you are shooting at the same distance the 400 dof at any given aperture is less, of course, but in your original post you said this "I did realize of course that with the 300 the runners are closer to me when I fire the shutter, vs. the 400, which could matter. "
Also, it is probably the CR forum re-size algorithm but neither of your shots look sharp, however, if on the "missed" shots an adjacent runner etc does have critical sharpness then you are almost certainly looking at a non optimal AF setting for the situation or a technique issue.
This isn't a personal attack, I have been using the 300 for years for surfing and yacht racing, my critically sharp keeper rate for those is 90-95%, even working from a moving boat. I have recently started doing dog herding trials and my critically sharp keeper rate is way down and I am not even moving! Changing lens can be just as dramatic a difference.
Keep improving your technique, I still am after 30 years, and play with the myriad of AF settings the 1DX has, Sportsshooter.com sometimes have people post their complete AF settings for a sport, I know the Sports Illustrated standard 1D MkIII/IV settings were published to give people starting points for different events. If you are not a member join, and ask some questions there, some of those guys really know their AF settings.