For me, I think the IQ is not as good if you are comparing it to the 800mm, and the 800+1.4. Or even the II with extenders. It would nessarily be a deal breaker, but I feel like at $13,000 plus tax. It should have at least matched the previous gen IQ.

But sometimes IQ is not everything, and of course I would rent any lens like that before committing to purchase.

I don't knock it entirely. But I am used to Canon producing stuff that has NO compromise compared to the previous gen. When you compared the 600 II to the 600. It just simply smashed it out of the park. There was no down side. It was much sharper with and without extenders, better contrast, less CA, it was much lighter, better AF, much better IS, and a more up to date design as a whole.

When you pay as much for the new version, you don't want to roll back on any features.

Now if my shooting style included a lot of hand holding or just simply didn't want to lug around as much weight. Then I would look past the IQ, and it is still by no means bad. Up until you slap the 2x. Then it really start to show some weakness especially in the contrast department.

Overall all, I'm just slightly disappointed in the 600 III IQ. But if/when I have the wifey approval for something like that. I would probably give the III a chance and rent both the 600 III and 800. I want to see in the field, how I use a super telephoto lens, under the circumstances I shoot. How noticeable the IQ is and see how the other benefits affect the overall picture taking process. Because sometimes IQ is not the only factor, maybe the better IS makes up for it depending on shooting style for example.