My friend took the plunge and ordered the new TC III and then we tested it using my 70-200 VR II and his 300mm 2.8. We compared it to the TC 14 and 1.7 versions.
I don't have the samples ready to go, but we found....
The new TC III was barely superior to the old version and had a focus issue. At close distances we had to use focus tune to get it to work. We ended up doing MF to get it right and quite frankly I doubt anyone could tell the difference between the old and the new versions. A 2X TC is just a LOT of magnification. We found it actually worked better on the new zoom over the prime, which was interesting.
A few other results I've seen online seem to be the same as what we saw. So for now I'm sticking with the 1.7 version, which was clearly better than the new 2X and quite close to the 1.4.
More reviews will be coming and sample variation may be a factor, but as I like to say....nothing is for free and a TC exacts it's toll to be sure. Thus far we could not conclude that the new TC is worth owning, unless you're fully aware of the loss of IQ.