I reasearched the differences between the ver 2 and ver3 TC for my 500mm f4 IS I and came to the same conclusion as the tests done in the link provided by Click. The contrast differences were very minor which I could adjust, if necessary, in post. The ISO 12233 crops on TDP also confirmed that the ver 2 of the 1.4x TC was slightly sharper than the ver 3. I couldn't justify the difference in price of about $200 for the ver 3 based on these test results and so I opted for the 1.4x II.
The ver 3 improvements would only be realized when using the TC with the 500mm II. My actual results confirmed my decision and I'm glad I did not spend the extra money for the 1.4x III.