With motorsports, there is a lot of variables. What type of motorsports? What is your access to the track/stage? What type of image do you hope to achieve? I usually do panning shots or corner shots in daylight with rally cars, and haven't had much issue with an E-M5. Pretty sure the E-M1 or GH4 would do better than the E-M5. That said, I can't say they are great in low light, but then, in Rally, you don't have a stadium full of lights.
That said, if I was trying to get a shot like D Fuller's, I'm not sure. If you could zone focus the spot, you might be able to get it, but using AF, it might not quite be fast enough.
Olympus has a 300mm planned (so 600mm) that looks to be pretty good, but not sure on release date (don't think it has been announced).