Sorted out the problem why 400 mm seemed soft. The focal length of the 100-400mm at that distance from the target was 370mm, compared with a true 420 for the 1.4x300 with the prime. The smaller focal took the size of the central rings of the chart below what could be resolved. So, I repeated the shots at 400mm (= 370mm) closer to the target so the image is the same size as that from the 420mm. The rings are now very nicely resolved (phew!). Here are the comparisons with my usual procedure. RAW, DxO 10, PRIME noise reduction, exported into PS. On the left there is zero sharpening, on the right 0.9 px at 100% USM.
I am much happier now.
...so at what distance did you actually take your readings?
If the effective focal length is reduced from 400 to 370 it looks as if it is at close range.
I'll check out the focal length at infinity soon.