I've got a 12-24mm HSM Sigma lens that I use. I find it fast enough for what I want. I mainly photograph bmx'ing, skateboarding etc. and it's more than capable of handling big photo sequences, 10, 20 pictures etc. I shouldnt think it would have trouble with motorsport. You'll need to use a tripod though for anything over 300mm. I used to have a 175-500mm Sigma lens for my EOS 5 way back when you had to pay to develop pictures! I ended up using it pretty much all the time on a tripod. Personally i'd say get the canon IS 100-400mm L series lens. Yeah it is a grand but you can get it from warehouse express and pay for it monthly if the cost is that big a problem.. Awesome lens.. That;s what i'm getting next.