scales, in a perfect world, we'd all get to have 500mm f/4's and TC's to boot. but realistically, money is money and I think a 400mm lens on a crop body is pretty decent for yellowstone.
I've seen not only bison and elk, but bighorn sheep, pronghorn antelope, and a fox, all within yards of main roadways (and pictures to prove it; although unfortunately I was using the nasty BigmOS that trip). almost all in Lamar Valley; that's definitely the place to be for wildlife. even stuff that's out on the grasslands, you'll at least have a fighting chance to get a decent shot at 640mm equivalent.
the other alternative is to get one of the big sigma zooms ... but other than the 100-300 f/4 (which doesn't give you much of a reach advantage at all), I haven't been satisfied at all with their image quality