Dieter Schaefer , May 15, 2009; 02:15 p.m.
Matthew, I can at least partially answer your question, I purchased the 500D specifically for use on the 80-400 (or 80-200) - zoom ads flexibility and so the 500D never finds it way on my 300/4 AF-S. To me, the 500D is kind of filling a gap - when I am out with the 80-400 and don't have the space or inclination to carry a dedicated macro lens around as well.
Doing a quick test - the FOV at 80mm with 500D is roughly the same as the one at 400mm without - at about half the distance to the subject - just eyeballing from a quick test. Seems to me that the 500D allows a (almost) continuous increase in magnification from what is achievable without it on the 80-400. Whether or not the optical quality of the combo is sufficient for you is a question I cannot answer. I use it under circumstances where the alternative is to get no image at all. I usually hand hold the combo - but focusing with the 500D can be tricky.
Here is a list of achromatic close-up lenses: http://www.angelfire.com/ca/erker/closeups.html
If you can find a Nikon 5T, you should be able to mount it with a step-down ring the 80-400 - provided of course that you use a DX camera. I have tried the combination on a 80-200/2.8 and have not seen vignetting due to the step-down ring.
I have not used Kenko extension tubes.
As to how useless is the tripod collar: I have added a RRS plate and it appears sufficient for my purposes - but the lens rarely gets used on a tripod anyway. If yours is on a tripod most of the time, then get a replacement collar.