My K-5 focused and tracked really well a snowmobile which was coming towards (not the easist job for camera to keep it's focus on snowmobile or any fast vehicle coming towards) for me. Took 5 photos and all of them was sharp ones. And the lens was DA 55-300, distance to snowmobile was maybe 5-10 meters, snowmobile's speed was something like 50 mph. So I don't see there's a problem with tracking subjects with K-5..
Here we go, five photos
Oh and I wanna show one wedding photo too, taken by me.
K-5 + Tamron 17-50/2.8 @ F/2.8, ISO 6400, 1/100, 50mm, maybe 3 meters away from me.