I love it when a user says "it's only one stop" while they forget its only one stop that separates the super-teles from L grade zooms.
Eh, no. Not hardly. Not even close.
Except for the not-yet-available-for-sale 200-400, the fastest 400 you're going to get in a Canon zoom is f/5.6. The supertelephoto 400 is f/2.8. That's not only two stops of light, it's the difference between Group A autofocus (all points doing everything they can) and Group E autofocus (no dual-cross points, no high-precision points, cross points only in the center, mostly just horizontal-only points) with the 1Dx and 5DIII. Even the 24-105 f/4 has better autofocus performance than a zoom that reaches 400. Indeed, the 400 f/2.8 with a 1.4x teleconverter still has better autofocus than a zoom with 400mm -- and no zoom will cover 560 or autofocus worth a damn there if you somehow kludge it.
And context is key, too. Few people doing telephoto portraiture are doing so in conditions so dark that they're ISO-limited with a 5DIII or a 1Dx, with the rare exception of theatre and concert photographers. And DoF at standard portrait distances is already so insanely shallow at f/2.8 (let alone f/2) that most portrait photographers are going to be stopping down to at least f/4 if not f/8 to maintain sharpness of more than a single eyelash.
In contrast, typical shooting distances of a Great White are much longer. Typically, you're taking a whole-body portrait if not even a group shot, as opposed to a head shot. By the time (before, actually) the action is close enough for a head shot with a Great White, you're dropping it and picking up your second body with the 70-200. Depth of field is still shallow, but you've got more room to work with at those distances. That, and you need much faster shutter speeds than you do for portraiture or even the theatre or concert hall.
There's a reason why you'd always shoot a Great White wide open on the field and often wish it was even faster, but few successful studio portrait photographers are often pushing the aperture limits of their lenses.
Don't be naive.
200mm f/2 - 70-200 f/2.8
300mm f/2.8 - 70-200 f/2.8 + 1.4 TC
400mm f/4 DO / 200-400 f/4 - 100-400 4.5-5.6
400mm 2.8 and upwards have no zoom equivalents in the range.
A stop is a lot. Don't belittle that fact.