It's probably more durable that the lens mount after attaching and removing a TC / camera many thouands of times. The lens mount on most large tele's take quite a pounding over time. This new built in TC should reduce that massively...and let less dust / sand inside too.
Yea, my thinking as well. Not to mention teleconverters, cameras and lenses get dropped in the swapping process. Not often, but I have seen plenty of pros do it. I watched a friend chase a 2X III converter down a hill as it sort of bounced and rolled away from him. He'd been trying to swap it into a 5DII 70-200 setup to get a quick 400mm shot and it slipped from his fingers.
If this lens were 10K, I'd consider it. I could sell my 400mm 2.8 IS and both teleconverters, and I suppose my 300mm is f4 as well. But I don't think it will be 10K, or less than it's retail price for a long time.
If the cost is an issue, hold back a year....let all the rich folk buy the 1st few waves and then pick one up for about 20% less. Sure, there'll be less hype and more known about this mysterious lens...but that's often a good thing. Who wants to be first? I don't, every one and their mother will want a go. The mighty 400mm f2.8 L IS mk II was quoted at £12999 (UKP) on launch....no it available for £10995 without even looking hard.
Oh and that's another thing, is it really better than a 100-400 L IS on one camera and a 400 2.8 on the other? If not, then I think a mk II prime is still the better option.
I see this new reviewer used it with a 2x TC, I'm wondering how it would fare with a 1.4x tc in addition to the built in 1.4x. In theory, it should be pretty spectacular and at f8....it'll still AF on a 1D series.