I just finished a week's catalog shoot using the Mark III for a good part of it. (1Ds2 and IIn used as well.) I do a lot of movement shots when I shoot, and I loved the extra speed in FPS. I didn't notice any marked difference in in/out of focus shots with the Mark III compared to the other bodies. However, perhaps this is because of my shooting style and how I use the autofocus.
I've found that all the cameras, at times, have difficulty obtaining focus in there's not a lot of contrast, light level is low-ish and I'm using all 45-focus points. Since 96% of my shooting is done with the camera in portrait orientation, I usually use on the top sensors and try to maintain the point on the model's face. Additionally, I never just "let 'er rip". I end up shooting quick bursts of 2-3 frames at a time. As such, I find the IIn no better, and sometimes worse, than the Mark III. The added FPS rate of the Mark III makes shooting action a breeze.
So my thought is, 10 FPS isn't too much for the camera, at least when using quality optics with updated USM motor.