5.6 is too slow for a prime....... like the 800-5.6?
Slow because you don't get the necessary shutter speeds? You will in good daylight. And using a 70-300mm on crop (=112-480mm 35mm) I can say that the depth of field with f5.6 is still pretty thin, so if I could with my crappy crop sensor I'd rather use f8 and higher iso often. So if the new 400mm has the new IS, fast af, is sharp, affordable and not that heavy it's a winner unless you plan on using a tc.