Feedback: I used the 1.4TC for a day, but when I reviewed the shots that night I could see some image degradation. I quit using it because of that, but I really did like the focal length. Basically it allowed me to shoot exactly as I've been doing except not have to crop. But I think a) it may be better to crop, and b) I'll just have to get closer.
I've been strongly considering a Sigma 150 macro, if I could get a port to fit.
As for focusing speed, I was correct. The D850 is faster enough than the D810 that a TC'd lens feels about the same on the D805 as the bare lens did on the D810 in terms of focus speed.
The loss of sharpness was very slight, but I'm chasing those gains.
Here's an example shot with the 105+1.4TC-EII.