Yes, I agree, the 70-200 is a hefty old beast but gives some great results. I think I would just get a cover all 18-200mm to carry on the bike and sacrifice some Max aperture, size, weight and cost so I could feel better about having an off! I have seen some fantastic motorbike photography using flash, especially as fill - but so far I have enough to think about with composition and motion! I think I will try some flash work out and re-post for advice here.
I agree with Tom that I have to get more of the section in with these top riders to show the real drama. Oddly it is the opposite with my riding - often better to see just a bit and hope people presume it is dramatic!
Thanks for the comments folks.