IMG_1491 by kayaker72, on Flickr
So, yes, I have. But I am going through something right now. I used the M6 II with the 500 and various extenders in April. My impression was mostly favorable. I found that it worked fairly well with the bare lens, ok with the 1.4x TC, but then not very well at all with the 2x with the issue being ability to AF. I gave myself several weeks of shooting exclusively with the M6 II to really get a feel for it. But, as soon as I switched over to the 5DIV, even with the 1.4x TC, and I was like...ahh, this is better.
But, given the Yellowstone trip (assuming it still happens, taking a second look at everything this weekend), I wanted to test out the M6 II with the 500 mm again to make sure I would be satisfied and....it is killing it. The AF is much better. I am hardly missing any shots. It is even more consistently critically sharp than my 5DIV.
I am not sure what the difference is, but I have two theories, either of which would say something about DPAF: 1) There is something about the quality/quantity of light effecting AF speed and accuracy. In April, I shot a lot with clouds, right after the rain. Here in July, still some clouds, but generally brighter; and 2) I am shooting subjects that are closer. Due to the leaves on the trees, I am having to wait for the birds to hit my feeder, which is ~30 ft. In April, most of my shots were in the trees, which range from 50-70 ft. So, either DPAF is more sensitive to light than PDAF, or it has more difficulty with smaller subjects. This actually plays considerably into my thoughts on the R5/R6 for wildlife. Granted those are DPAF II, but I know AF speed for PDAF speed is sensitive to brightness and am wondering if DPAF speed and maybe even precision is very sensitive. I'll work on that at some point.
I should also note, in April, it could have been user error (). While I was shooting 1/800th-1/1250th, etc, that may not be enough for 1.6x plus, 1.4x = 1,120 mm equivalent. At the time, I was thinking "IS", but I'll shoot more today at 1/2000th or faster. This high of pixel density will likely pick up everything.
I have been shooting only at 500 mm in July as the feeder is closer and, honestly, the birds at 30 ft fill up a good amount of the frame, especially the WP. But, I'll add the 1.4x TC today and post some results.