Nebraska,
I had some thoughts on options for you;
1) Rent or borrow the extender to try before you buy. Make sure it's a MkIII. You might find it works for your purposes.
2) Rent the long lens when you need it.
3) This one is a stretch...but the Sigma 120-300 f2.8 sorta covers the range of both lenses (yeah, you lose the long on one and the short on the other). It's a beast (big and heavy). I've shot it handheld with a 1D and I won't be doing that anytime soon again. It's also pricey. But I think it's less than a 70-200 f2.8 + 100-400. The sport version has a very fast autofocus and is sharp. ...again rent to try before you buy.
