The thing about 4/3 is that the sensor is smaller than other digital SLR cameras, so the pixels will be smaller possibly causing some of the issues mentioned in the first post.  You may also find that the lenses with the fast focus motors may be expensive.  I've been away from the market in general for a bit now though (used to be involved in it until about 4 years ago) and there will have been advances in controlling image noise.

However, with 4/3 because the image sensor is half the size of a full frame camera, a 200mm lens will act like a 400mm so carrying the gear around will be less of an issue.

All that said, there's a reason why most motorsport photographers use Nikon or Canon and that's because they have the tools for the job.  I've never met a working 'tog who wanted to lug half a hundredweight of gear around but the reason they do is because the smaller, lighter stuff just doesn't get it done.