I'll agree with JM on the new 70-200 OS, I'm pretty impressed with this lens, it would have to be the best bang for buck 70-200 2.8 on the market, I found it to be a great deal better than the non-OS version, after saying that, I see you want this lens for motorsport, my preference would be canon's 100-400, having that extra reach can come in very handy, especially when you can't get good access to the track, and because you're usually using slower shutter speeds, 2.8 isn't really necessary, I miss not having one anymore when I'm at the track.
In regards to the 24-70, I've only ever rented the canon, and I'm not sure if that lens was a so called, poor copy, but I can honestly say my 17-55 was better.