Hey Mike....sorry for the delayed response. My mornings have been taken up with running my wife to her conference, etc. leaving little time to post on the forum.

So, any more thoughts now that you've had more time to look at the pics? 1Dx vs A6500? 100-400 II vs 150-600?

I have only been able to look on the 5DIII viewfinder, but I have paired the ~1200 shots I took down to ~600 in the first cut. One thing that I noticed in doing so is hat a lot of my shots with the 100-400 II were out of focus compared to the 150-600S. I do not so much attribute this to the lens (although one was AFMA'd and the other not, etc), but more going back to one of the benefits of reach, AF. I think the bird heads were larger at 600 mm compared to 400 mm not only benefits "pixels on target", but also allows for a larger target for the AF point, allowing for more consistent AF.

As for the M3. I still like the IQ coming out of it. But I tried it on some ibis walking around the condo the other day (adapter plus 70-300 L). AF on moving targets was not good. I tried all four combinations of AF point selection and servo vs one time AF. Part of it may be the ~0.1-0.2 sec shutterlag vs 0.5-0.6 sec for the 5DIII. Fortunately I intend it mostly for static subjects.