I agree the 1D3 is pretty solid, and once you go there there's no going back. Only knock I'd give it is for highschool ball you go to some pretty dark places and the 7d iso range is much better in that regard. I've got a 1D3 if the OP wants to go that way. I think glass is still the low hanging fruit here and obviously something within budget is the way to go. Dude is right basically about the 1d3 v 7d thing though it's probably a horizontal move not an upgrade and not really what you were asking but, there some validity there though.
This fan boy thing for the 100-400 is just crazy though. If you want a slow, softish and small aperture zoom get a 70-200 and put a 2x teleconverter on it and then at least you can have a top shelf lens some of the time. If you need any validation on that just scan the sidelines at pro and D1 sports or the Olympics. You don't see long white lenses with black hoods, they are all white hoods, black hoods are hanging on slings. That's just reality. Anecdotally pulling a few shots out of your catalog doesn't make f5.6 2.8 somehow, that aperture has utility. Maybe on the steppe of Alaska it's not as important, maybe things are spread out over the plain and separation is easy but, the OP is shooting clusters of runners with challenging backgrounds. His needs might be different.
