I decided to try out the AFS300 f4 + 2x TCIII on my D700 just to see how good the IQ was and also whether the AF would be fast enough.
Firstly, AF is quite slow, even in reasonable light. It will lock, but it is just a tad slow, so no good for fast moving subject matter.
Secondly, the IQ is quite amazingly excellent, sharp as a tack, colourful, quite contrasty and with few "aberrations".
In a pinch, using fast shutter speeds and shooting static or slow to move subject matter, you can get some exceptional results from this combo. I would suggest using a monopod as well. I hope these are of value to any potential purchaser of these products.
D700 + AFS300 f4 + 2x TCIII @ f10 (f5 2 x convertered), 1/1250sec, ISO1600, handheld.
Crop of above:
D700 + AFS300 f4 + 2x TCIII @ f9 (f4.5 2 x convertered), 1/125sec, ISO1600, handheld.
Crop of above. There may be some very slight camera shake due to the slowshutter speed and 600mm focal length.
D700 + AFS300 f4 + 2x TCIII @ f10 (f5 2 x convertered), 1/12000sec, ISO1600, handheld.