I briefly owned the 1.4x II and now have the III. The former showed CA with my 135L and my TS-E 17mm, and the newer one doesn't. Unfortunately, I am well behind archiving my photos so can't pull out examples too quickly.
I haven't seen claims regarding sharpness and contrast, though, and haven't specifically checked myself. I believe Macguyver said they are quite at par optically unless you are using one of the great whites.
Hope it helps.
That's right - on most lenses (e.g. the 70-200 f/2.8 II) the differences are very subtle, with the III having just a tiny bit less CA but on the series II big whites, you can see more improvements in CA reduction and the contrast is noticeably better. Sharpness is slightly improved as well, but that probably has to do with the reduced CA and improved contrast. The AI servo performance is improved with the series II lenses as well because of the mk III extenders ability to use AFMA calibration and faster CPUs for better speed. I didn't notice much increase in speed, but the AI servo accuracy is much higher. The distortion is also somewhat lower with the series II lenses as they were designed together with the idea that the distortion would essentially be cancelled out when they were combined. This is also the case with the 1.4x II and the 70-200 f/4 IS - when combined, the essentially have 0 distortion - according to conversation I've had with DxO's engineers and what I have seen with my own eyes.
I have also noticed improvements on my 180L macro photos as you can see here:The Digital Picture 180+1.4x II vs 1.4x III
and the best comparison is probably this one, even though it's not on a series II lens, the 200 f/2 is still one of Canon's sharpest:The Digital Picture 200 f/2 IS +1.4x II vs 1.4x II
I guess the final word is that the improvements are subtle, but worth the money if you have invested in the very best lenses. If you don't the money may not be worth it. The improvements on the 2x II vs III are more substantial.