As I have said before here - I shoot the mkIII. I don't mean to sound like a broken record but its worth identifying the model when describing a situation.
I have been shooting that camera for about a year now and it is just like it should be with amazingly fast and accurate focus. I use it primarily for sports and I shoot the 70-200F2.8 as my main go-to lens.
But I swapped my Pentax 300 F2.8 for a Canon 300 F2.8 (non-IS) a few months ago to shoot primarily for surfing with a 2x TC - and it performed very well as expected. Last weekend I took this combo out to a mountain bike race for the first time and I was blown away. Out of 408 photos only 7 were not amazingly sharp and at least 3 of those were user error. I shot the lens exclusively between F2.8 - F3.5 with most of the shots at F2.8.
To further explain - these can be very difficult shots to get because the changing trajectory can fool the servo motor. I used to find that on some bermed corners the success rate with 70-200 would drop dramatically. I don't know why my success rate was so high and perhaps as I get to use this gear I will find weaknesses but for now I am officially impressed.
These images were all shot as JPG
ISO 500; F2.8; 1/1600
ISO 400; F3.5; 1/6400