Alex, thanks very much for the info! In the low light conditions and woodland shade, etc...was that in servo AF mode? And how many AF points were selected on the various bodies?
Would you care to post a cropped version of one of your sharpest shots with it? If you have already, I apologize for missing it.
When you did the AFMA, did that remain the same throughout the zoom range, or was the adjustment only good for the long end, etc? That would be my biggest concern.
Oh, and at what point does it become f/5.6? At 200mm, 250mm, 300mm? Hopefully not below 200mm...
As for similar sized lenses, I've only rented the Canon 400mm f/5.6 prime, and the 300mm f/4. (I've rented a couple of bigger superteles also). I found the 400 to AF extremely well in very low light, 10 to 20 minutes past sunset (depending on if there were diffuse clouds, or if the sky was clear). I was using center point only, in single shot (not servo) AF mode. Of course to get decent shutter speed, I was too high into the ISO, but it was fun to try anyway.
I found the 400's sharpness to be extreme, and contrast very good. Closing it down up to about f/7.1 only helped a little, and actually began to reduce what I would call global contrast...as most all lenses do as you close the aperture (sometimes that's an advantage though). Using the Canon 2.0x ii TC, looked a bit better on the 400, than on the 300. Of course I had no autofocus with it connected to the 400 on my camera, but it did work on the 300 f/4, mounted to a 1D Mark 4. I had no 1.4x TC on hand to try, but I have no doubt it should work superbly on the Canon 400 f/5.6, especially on a full frame camera...provided you could make it autofocus.
Obviously the faster superteles will go sharper and have better color and contrast, will provide a less noisy image with better bokeh, and will AF much better...but you do pay a lot for all that, and they're bulky.