Using either C-AF or C-AF [TR] modes, the E-M5 is capable of some decent motorsport images at 4fps.
Not any faster, because you can't use continuous AF with the high speed (5-9fps) burst mode of the camera.
There is a trick I've found to successfully using the C-AF modes; ignore the camera manual.
In my experience over the last two days, half-press of the shutter to start the tracking mode AF is a waste of time. The tracking box glows green and jerks about drifting off the target (the driver's head usually) and then a full press results in about 3 seconds of not taking a picture ... by which time the car it out of the corkscrew and on the way to turn 9.
I found the same for a vulture in flight; start out with the box on the vulture, half-press and the box jerks about and loses track of the bird then the focus hunts and the bird vanishes as it goes so far OOF that there is not even a dark blob to track.
Seems that the tracking works best if I half-tap to set the AF, then track the subject with the [ ] black corner markers round the AF point, and when I want a photo just full-press and hold it. No half measures just mash the button down and off it goes.
The Af is really fast for the first image, then a 1/2 second pause maybe (not always) then the burst continues with focus tracking.
It lives up to the claim of a fast AF system for single AF, and for the first of a continuous focus session - then there is a delay before the rest of the burst. I need to find some way of recording it to show it happening.