Canon 100-400mm is not really a daddy when it comes to motorsports. In fact I have never seen any full time staffer use one. Even the two guys I know who have one refer to it as the canon dust pump. For that kind of money save up more and just get a 70-200mm F2.8 IS2. The 70-200 can be used for taking awesome driver portraits, pit lane, the gird walk and race action. Where as the dust pump would only be track-side use and its not even very good at that considering its a F5.6 at longer focal lengths. With the 70-200mm you can snap on a 1.4 TC and so long as you stop it down a little it will still return sharp shots with little degradation to AF speed. I would not recommend anything more than a 1.4 on a zoom lens. If you need longer optics I would opt for a 300mm F2.8 but I would take a guess that's way out of the budget.
70-200 with 1.4 TC (before I owned a 300mm F2.8)
