Why would anyone be enthusiastic about an f/5.6 zoom lens with a built in TC, especially one that is still large but limited to 300mm at the wide end? F/5.6 is dark enough...an f/8 lens is limited in its usefulness, even if by some miracle future pro AF sensors will work with all their points at f/8. It's still a very dark lens...perhaps useful on ski slopes during bright sunlight.
What should get built instead (or at least...first), is a smaller and more affordable, light weight prime lens, but with the TC built in. How about say, a 330mm f/3.5 DO (with some major technological breakthroughs in resolution), that weighs 3 pounds or less, and has an option to switch out two different TC's that then mount internally? Maybe a 1.4x and a 1.7x? Canon could still charge $3500 to $5000 for it, and lots more people could justify buying it. It would be highly portable, hand-holdable, and usable for long hikes, or a long day shooting at an event, etc. I say it would be more useful to more people, than a $10,000+ 300-600mm f/5.6 zoom (which is basically a very similar sized lens to the 200-400 f/4...which itself also seems more useful than a 300-600 f/5.6).
Most importantly, it would be a light bucket by comparison, at f/3.5 and 330mm...yet weigh half as much, and cost half as much! Then of course its AF speed and accuracy, could easily exceed that of all but perhaps the 300mm f/2.8 ii...I suppose if it did all this Canon would charge closer to $6000, but it might be worth it! It could be nicknamed the "mini mighty whitey"!! Hahaha...
I just have to think the AF speed freaks, would look down their noses at a 300-600 f/5.6 zoom.
What's next, a 10mm to 100mm fisheye zoom??