I wish I could show you the quality, but the file sizes are 350MB when scanned and when I JPG them down you lose the quality due to compression. Remember, since I'm shooting film I lose a bit from the scanning (Nikon 9000).
I don't personally notice a quality difference between any of the lenses, but a better trained eye may. There is something special about the 120.
To my surpirse, the 1.7x Extender is very sharp on the 300mm. I did not expect this to be the case! I just returned from a 2+ week shoot in South Dakota and I used the 1.7x on the 120mm but I have not gotten to those shots yet. I can tell you the gear stands up well in the dust, dirt and elements.
Do you mean extension tubes? I have only used the extension tubes on the 120mm, so I can't give you a comparison.
I have considered the 100 f2.2, but I don't notice the need when I have the 80 and the 120.
Derek.. How is the sharpness and optics of the 120 compared to the 150 or the 210mm? Have you ever used an extender on the 150 or 210 to compare with the macro?