Congrats and nice write up. Since you brought up the 400 DO II thought I'd pipe in. Sorry for going off topic but your input will be helpful.The 500 was my dream for years but as time went on and I got older the 400 became more appealing. I wish Canon has offered a 500 DO. 600 is rumoured so maybe an upgrade one day.
First of the sharpness with a 2X has exceeded my expectations. What I have found so far with AF. The 1.4 is a cake walk but the 2X is the challenge. When use AI Servo, put the AF point on a static object, press the BBF the AF speed is more than adequate despite the 75% delay the 2X causes. Very acceptable. Even after moving the AF point to another object and back it is pretty good. I tested with TS at -2 and TS at +2 and AF release response times worked as expected between both settings.
BIF. Hard enough to track a blur at 800mm and it takes forever to focus. I pretty much have to pump the BBF when the AF point is on the target. I'm OK with that because and there not much I can do about it. No fine tuning settings for contrast detection. Works great on static objects but I guess that delay really kicks in for moving targets. The delay makes me wonder how it effects tracking. Does the delay only effect initial AF acquisition or tracking as well? Did you notice anything?
My keeper rate went down which I expected. A lot due to me. Even with IS on you can see any slight movement at 800mm. You need to be steady. I always come home with something so I'm not worried. I have not tried the pre-focus option so I'm not sure how much that will help. Gotta have something to pre focus on first. Not tried without IS but can't imagine not having it. I'll give mode 3 a run.
I also asked Ari this question and just posted it but I think it needs to be reviewed. Kinda wondering what your past and current experiences where. I got it to get more reach for wildlife in general. I wondered about BIF at 800mm in general but results came quicker than I thought it would.
From what I know, and have experienced, only the 1DX II drives the TC III's at full speed with no slow down, whereas the 5D4 (maybe others, this I don't know) drives the 1.4x at full speed, but reduced speed w/ the 2x. But I most definitely notice reduced speed and "sure-footedness" using the TC's, especially when shooting in places with distracting backgrounds (such as tree branches); I'm actually not too experienced with BIF, so my observations may not apply.