Well, I finally did some comparison tests with the 100-400, at 2 different focal lengths (349mm and 400mm), 2 different apertures (wide open 6.3 and f8); and also adding the MC1.4 TC (500mm and 560), 2 different apertures (wide open f9 and f11).
I was as far away as I could get in my house, from the back of the kitchen to the living room window (maybe about 40 feet). My test subject was one of those enamel tin posters you can buy in stores and the internet. They are very detailed and sharp. This is of the Titanic (matching the size of the 100-400 lens!). I used a tripod and a wired remote shutter release, ISO 200. The poster itself:
View attachment 865079
Below are the results, in 100% crop size.
349mm, f6.3
View attachment 865080
349mm, f8:
View attachment 865081
400mm, f6.3:
View attachment 865082
400mm, f8:
View attachment 865083
500mm (with TC), at f9:
View attachment 865084
500mm, f11:
View attachment 865085
560mm, f9:
View attachment 865086
560mm, f11:
View attachment 865087
My initial impressions:
1) at ALL focal lengths, it helps to stop down 2 stops from wide open (it probably also helps to stop down one stop, but I didn't try that).
2) there is not much difference in sharpness between 349 and 400, using f8. So not much point in backing off on the zoom. Also, the worst results I got was at 349mm and f6.3, so backing off on the zoom doesn't help when you are wide open either.
3) the TC doesn't hurt much, although it seems that backing off on the zoom some can help. I think that 500mm at f9 or f11 is essentially as sharp as 400 (without TC) at f8. But even here, 560 isn't bad at all, I think. The TC results are confusing in that sometimes the wide open results at f9 look better than the f11 results. I was pretty careful taking these shots, so I do not think there is any camera shake.
Anyway, I think I will be stopping down some (especially when not using the TC) and that also I won't be reticent about using the TC; it's pretty fine, indeed!
Hope this helps. Your results may vary!