Curious about the maximum magnification. B&H lists .15x, the same as the 400f2.8, but it has a much shorter minimum focus distance than the 400f2.8.
The 70-300L and 70-200/2.8L IS II both have the same MFD and maximum magnification, despite the former being 100mm longer. Since focal length is specified at infinity, the 70-300L must have a lot more focus breathing, seems the 200-400L does as well.
It's a common issue with modern tele-zooms. It's one aspect (along with increased flare and bokeh nevousness) where primes tend to excel over zooms.
The same focus breathing also happens with the Sigma 120-300 f2.8 OS to a large amount, about 20% taking it's real focal length down to around 240mm at MFD. The Canon 70-300L looses nearly 1/3 of it's focal length at MFD and is quite astonishly short....but it's IQ at that close focus is extraordinarily good, far better that my 70-200 f2.8 L IS II. I guess every lens is compromised somewhere in the design path.
The new 200-400L has a few design questions, Canon have been very coy about releasing it's MFD / MM specs until now. I'm wondering if someone was to pop this new lens on a test bench and work out it's real focal length at MFD would be shockingly low. It may have the same MM as a 70-200 f2.8 LIS II, but a user would be twice as close to achieve it unless something is really compromised in the design in this particular spec.
I think it's not enough to discount this lens as an admirable optic and one which can offer some serious wildlife and spots photographers a real world advantage, especially in harsh conditions. I just think it's not possible to create a "it has it all" tele lens. Lets face it, it's quite heavy, large and expensive compared to the new 500mm f4 L IS II. But it's a lot more versatile.
I seriously doubt that this new Canon version isn't very different to the Nikon variation in this respect.