First off, the riders will love the photos. I'm a rider too, so believe me they'll like them. lol
I noticed you have your camera in "Shutter priority" for panning. Now try another way. The 6D and 70-200 F4L are a great combo for motocross.
Give this a shot. Set your camera to AV (Aperture priority) ISO 200, F4, Auto focus mode set to AI Servo, Auto white balance and Select the single point AF point. (use the center one) The camera will pick the shutter speed for you. You'll see shutter speeds anywhere from 1/800 to 1/8000. That's going to stop all motion. (that's a good thing) Some people will say that you need wheel blur to make the photo "interesting" well, that's a load of crap. If you capture the right action, the wheels will not matter.
Now, set your 6D to use back button focus (video link right below)
The whole time you track your subject hold the back button down and your camera will track the subject so all you have to do is press the shutter release. It will already be metered, and the focus will stay locked on the subject. (If you use only the shutter release your camera will lock on to the subject and by the time the shutter releases it will be out of focus again)
I know it seems like a lot, but it's not.
Lastly, if your shutter speeds are too slow (1/250 or below) increase your ISO until you're above 1/320-500 ish. (that should only apply on overcast days)
Seems like a lot to digest, if you have any questions feel free to ask.
Eric