Hi CarlTN, on the 1DX my 120-400 seems a bit soft at the long end. I've noticed that it's sharper up to about 330mm on the 7D and then loses a bit, so a bit hit & miss. That seems to be exacerbated on the 1DX, no idea why. I've never noticed it being particularly slow, but perhaps ii isn't quite perfectly locking on. My new 120-300 is so far giving me a much better keeper rate, including compared to my 300f4L.
I'm not a pro but have had a couple of drivers offer me money for shots of their cars. (Can't retire on that income yet, though!) I started out just going to the classic/historic car meets (love those machines, many of which I watched in their prime) but have branched out to other classes as I've got to enjoy the challenge of tracking cars at speed. Modern tin-tops are also colourful beasts and make good subjects. We've got a circuit 80km away which is ideal for shooting, with banks slightly elevated above the track - no blankety-blank heavy catch fencing getting in the way. I'm off there again this weekend.
Have also shot rallying, which has been my primary passion for the last 30 years - you can get a lot closed to the action and a bit of dust and gravel flying adds to the shot!.