Lucky for you, the October UK issue of
"Digital Camera World" has an article on shooting motorsports...
To summarize, while a 100-400mm lens is nice, they spent most of their time at the 200-300mm range. They also claim that f/5.6 is good enough, provided you pan (which adds to the picture anyway) and use a monopod (which they seem to feel is a must-have).
Hmmm... I get the issues late, being in the States... that issue might already be gone and the November one in the stands...