I think this might be of interest. Bryan posted an IQ comparison between the old/new Canon teleconverters. The test lens is the 200mm f2 L IS which is apparently very sharp:
1.4x III vs. 1.4x II
2x III vs. 2x II
As Bryan pointed out, the new 1.4x is similar to the old one but with less barrel distortion. The new 2x though looks much better.