I have just been testing the mkIII 2x tc against the mkII and my initail impression is that shooting with a mkI 300 f2.8 and a 7D there really is very little to choose between them. I did some casual shooting of some garden birds but couldn't shoot the same subject with both so set up a really dull test shot. I taped a £5 note to my door and shot it and relatively close distance. I shot both 2x tcs and also with stacked 1.4x (a Canon mkII and a Kenko pro 300) at a series of apertures. All the options improved when stopped down a bit but the difference in quality wide open at f5.6 was interesting (similar difference where seen at various f stops). Attached is a 100% crop from each set up (I took a few shots with each planning to choose the sharpest but in each case all were of a very similar standard). Attached are the three samples, I haven't labled them so would be interested to see which people think is which (no peaking at the exif)!
Full size files are on flickr if anyone wants to see them -
http://www.flickr.com/photos/37669825@N04/sets/72157633240341249/ for each of the options there are shots at f5.6, f6.3 and f8.