Hey Alex,
I never seen a 300 + 1.4 on a Rebel before. I bet that was a handful.
Your shots are nice and crisp. Good going there.
Normally you want to slow the shutter speed down and pan with the cars. The Flying Lizard Porsche you almost got it, but the rest of them are a little too static. You can get away with that if you do some real tight crops and you don't see much of the tires or the background. Usually around 1/160 or slower will give you more motion blur and depending on the speed of the cars, usually 1/250 will blur the wheels fairly nicely, but not hide the background as much.
I used to have an XT and it couldn't lock focus quick enough and the frame rate too slow so I know what you're going through.
So you borrowed or rented the 300? If so, it would have been hard for me to return it.