I don't have experience with that lens, but you're trying to cover a huge focal length when it might not be necessary? For motorsport, I tend to use a 70-200 F2.8, but I suppose there are some scenarios where you may need a larger focal length. If I were you, I would look to hire a good/fast constant aperture lens. For example, a 70-200 f2.8 ISII (and an 2x extender if you feel you may need it...). Speed isn't everything, as you may turn to manual focusing, but a lens like that can be picked up for the weekend very cheaply and covers AF/MF very situations very well. Just a thought!