Uhhhh, why the heck would you use 2000 or 4000 shutter speed? Redo the test with the shutter at 1/125 and it should look fine, the results I got looked as expected. It's possible that those cars are just moving too damn fast for 60fps, but I think that shutter speed is just way too high either way. That kinda sucks because they could have been really really cool slowmo shots.
Also, do you have FCPX? If you do a standard retime in there it's the same as Optical Flow in motion and much easier to get done.