I can relate my experience in testing the 400 5.6
I have been shooting for a while with a Canon 300 f4 with a Canon 1.4 III extender attached and last summer I had thoughts about buying the straight 400 5.6
My thinking was that a similar size and speed lens would possibly have better IQ and faster AF then a lens using an extender.
Before buying I was able to test two different 400 5.6 lenses. One was new right out of the box and the other had some use.
I tested them on two different 7D bodies and a 50D body.
I did tests in the field on birds and wildlife (which is what I primarily shoot) and tests on stationary subjects under more controlled conditions.
I was hoping to be wowed by the 400 5.6
However, my results were that in nearly every case images from the 300 with 1.4 were sharper with better detail then those made with the 400 5.6 and AF speed was roughly equivilant. The 400 5.6 may have had a small advantage in AF speed. But I'm talking small.
It got to the point where I could immediately tell which lens took which shots just by looking at them. Enlarging to 50% only confirmed this impression.
Keep in mind this was a lens I really wanted to like and I was fully prepared to buy it if I was happy with the performance.
So needless to say I did not buy one and continue to use my 300 with 1.4 with what I feel are excellent results.
In the future, should Canon come out with a new version of the lens with updated, and upgraded optics and focusing (and maybe IS) I will revisit the 400.
But for now I'm staying away.
I know there are photographers that have gotten very nice results with the 400 and are quite happy with it.
What I described was my experience.
For examples of birds and other wildlife photos where virtually all were taken with the 300 4 and the 1.4 feel free to check out my photo blog which is updated daily.http://phillanoue.com