Here's the true test ... upscale the photo from the 400mm to the same magnification as that from the 800mm ... now compare them.
I did this with a Sigma 600mm f/8 Mirror and the upscaled photo from my 400mm was "sharper" and contained more detail!
Download the carpet shots, those are both with the 5D2 and compare the 400mm and 800mm lenses.
Here's some fresh shots with the T3 on the 400f5.6 and 5D2 on the 800f8. Note that the T3 is equivalent to a 31MP full frame, cropping off the 5D2 definitely loses more detail which is why the carpet shots don't seem as close. Both shots here are 1/1000sec, ISO 1600 and live view with a remote release.
Maybe I should have waited until I could get a shot with the sun shining directly on the shed with a 1/2000sec shutter speed, but I'm happy with the shots as is, the image off the 400f5.6 is pretty crisp.
It's pretty much the same situation as last time, sharpness is nearly tied, but the 800f8 has the edge in a few spots. It's just way harder to manage the DOF on that lens, and it obviously loses a lot of color and contrast.
I probably should have run the T3 at ISO 800 but the image off that lens already looks less noisy anyway, and details look pixel perfect. If anything this shows that the 400f5.6 still has sharpness to spare and should work well with even higher pixel densities.
I can't wait to stick a 20MP+ sensor behind that thing.