Key challenges I thought were trying to second guess erratic movement, without just resorting to praying and spraying at 12fps and where to position around the pitch. Headers it seems best to pray and spray at the moment!
400mm is a great focal length as it reaches both ends, I was able to shoot the team's keeper for example and crop in fine.
Ended up using single AF point with expansion of 8 points and Case 4 with Canon's new AF system, towards the top of the AF matrix in portrait mode, which seemed to work quite well. AF lock of the 400 is stupidly fast.
The best thing is that worrying about ISO is almost irrelevant with the 1Dx!