Lovely sharp pans
Only criticism is the colour/white balance and the framing, I suspect this is due to your metering mode, i find mine gets all confused if i shoot slow shutter stuff particularly with bright/dark coloured bikes.
Same lens i have, i would be very interested to see your results with the teleconverter. I had one once and it was awful, gave it back. It also came with dirty contacts and as such wouldn't communicate properly with the lens. Worked fine once i cleaned it. (check on your photos with it at 420mm that it reads 420mm not 300mm)
I prefer shooting bikes, dont find them easier, they scare me more as they come over barriers alot easier and tend to have quieter accidents. But they make for much more creative shots i find.