Thanks for the replies. I was almost sure neuroanatomist might chime in first since your posts are always very detailed, technical and well explained and you know the theory of many subjects discussed here very well. Thanks.

OK, so the 600/5.6 can be forgotten as it would certainly be too expensive and with the added flaws of no AF with extenders. The point about Canon not spoofing their own AF system with f/6.3 lens that reports as f/5.6 is fair and that was kinda "wishful thinking that won't ever happen" on my part. Also the fact that the newer bodies wouldn't AF with extenders is probably a big reason for not bringing f/5.6 superteles to market at all.

It is true that the 200-400 f/4 1.4x would be in semi-direct competition with a 500/5.6 lens but that is only if the price would be under 7K$ that I highly doubt considering the prices of the new superteles.

I wouldn't consider the slowish aperture of 5.6 to be a problem with the ISO performance of todays bodies even in dusk/dawn light.

So the 300/2.8 + 2xTC would be the "real" direct competition for my imaginary lens then. That combo is around 7-7,5K$ at the moment. So if there was a lighter (should be quite a bit lighter since it would need smaller diameter glass, right?) 500/5.6 version that would have faster AF and presumably even better image quality (used without extenders) at a price of $5K or even under that would make the hard choice of the superteles even harder. And what about a 400mm f/4 IS (100mm front element) for $6K?

Maybe Sigma will see an opportunity in this...

In the end I will probably have to settle on dreaming and waiting for a new 100-400 with modern IS and weather sealing or a stellar 400/5.6 IS to replace the old non IS version. Or go with the just announced Sigma 120-300/2.8 OS + 1.4x and/or 2x when it becomes available to get into the supertele focal lenghts.

PS. It would make a family portrait of the supertele L's even more impressive so it's an interesting thought exercise anyway