Thanks all.

Looking over that very long post, it sometimes seems as though I'm just glossing over the benefits and saying quite a lot about the drawbacks of big glass.

And yet ... what do you say about the main strengths beyond just listing them? Once you have said something like "image quality is near enough to perfect as makes no difference" (which is true, unless I stuff something up, and then it isn't the fault of the equipment), then there really isn't anything much to add. And, in any case, that is no more and no less than you expect as mere routine from any product in this price class.

One thing I could have emphasised a little more is that although you can't get a lot of extra reach and the full benefit of wide apertures and fast focusing, you can always get at least one or the other of those two main benefits: faster than a Bigma or a 100-400 for about the same reach, or more reach for about the same speed. And, of course, you are free to pick which of those two benefits is going to be most useful to you at any given time, simply by adding or removing the teleconverter.

Like most people, I started out using the converter almost all the time, but as time has gone by I have gradually swung over to favouring a bare lens more and more. Sometimes, I think how nice it would be to have an 800/5.6 so that I would never want to use a teleconverter and always have the optics unimpeded. Then I remember the astronomical cost of that lens (it makes a 500/4 look cheap) and the extra weight and the great difficulty of handling it (especially inside a car, which is where it would be most useful), and decide to leave it go. At least for this year.