Interesting theory Ken. But isn't mounting a TC on the back the same thing as "adding a group or groups that are swapped in and out to change the final focal length"? It's just that one is done externally and one is done internally. I do agree that since it's an integral TC, the optics can be tweaked for what they're mated to, so you should see less IQ degradation than you would if you swapped in a generic TC. And to an earlier comment in this thread, I'm sure that will be a test that we will see shortly after this lens is released. Shots w/ the integral TC "in place", vs. those same shots w/ a 1.4x III attached and the integral TC "disabled". I am genuinely curious as to what the results will be. Should be interesting.
They're not exactly the same.
What you may have here isn't necessarily a set of elements that's either there or not - it may be two separate set of optical elements, one giving a 200-400mm focal length and another, slightly different set, giving a 280-560mm focal length. If this is the case, the elements would be an integral part of the lens design, and there would be no IQ compromise in using the lens as a 280-560mm f/5.6.
Why would you want to have to add your own TC? To do so, you'd have to dismount the lens, add the TC and re-mount it; in the meantime, you may have missed the shot. The way they've designed it, you can keep it at 200-400mm to take full advantage of the f/4 maximum aperture; whenever you need to go longer, just flip the switch and you can get to 560mm f/5.6, take a few shots, then quickly flip back as the animal/bird/plane/whatever comes closer. Or, with a 1D-series body (or a nonreporting TC), add another 1.4x TC to the back and use it as a 280-560mm f/5.6, with the option of switching it to 400-800mm f/8 on occasions, should the need arise. (If you're shooting in the 700-800mm range consistently, you should probably be using a 500L with 1.4x TC, or 800L anyway).
I would have thought the Sigma 120-300mm f/2.8 OS would be the ultimate field sports lens. If you wanted anything much longer, while retaining f/2.8, you'd be looking at the Sigma 200-500mm f/2.8...