So, flying in a bit under the radar, IMO, given they were announced at the same time as the EOS-R were the new 400 III and 600 III.

I just saw Bryan post some IQ results for the 400 III. It would be difficult to gain in IQ over the II version, but considering the III is 6.25 lbs, down from 8.5 lbs for the Mk II, appeal of the lens is pretty obvious. You have to account for some copy to copy variation, but the IQ of the 400 III looks good:

Maybe a touch more vignetting than the Mk II. But even stopped down to f/4, that is basically perfection on a 50 MP sensor.

I like to look at superteles with TCs on them as, 1) that is how they will be used, but 2) it is likely to accentuate any flaw.

1.4x TC:

2x TC:

At least in the copies Bryan tested, I would give the center by a little to the Mk II version but the corners to the Mk III by a good amount.

Tough to compare to the 400 III plus 1.4xTC to the 500 f/4 II as they were not tested on the same camera, but we can compare to the 600 f/4 II:

It is amazing how something can look very good, but then you compare it to something like the 600 f/4 II and the subtle issues become apparent.

Yet, I am sure there will be many amazing images taken with the 400 f/2.8 III and 1.4xTC combination.

Anyways, some quick fun.