Gentlemen, this was never designed to be a lab test for sharpness. Turn off the OCD filters for a minute and think about his. A fellow photographer offers you a chance to shoot with a lens for a few minutes, and you take them up on it. It's a lens you were considering purchasing prior to buying the one you have. You spend 15 min. (or less) swapping lenses on your body, and take shots of the same things with each lens. Same camera settings. Then you go view the results and this is what you see...
What conclusion would YOU draw?
I call this a very fair test, as I used both lenses EXACTLY the way I would use them in the real world. Hand held, with the camera settings on my custom 1 mode that I shoot for wildlife (same settings that Arthur Morris recommends for the 50D).
The 100-400L has a 2-stop IS. The Tamron does not. I don't have any idea how to account for the difference in aperture or shutter speed, since they were taken from exactly the same position, just minutes apart on a sunny day, mid-day. It's not like the light was changing quickly, or at all even.
I know most here, myself included, have probably spent hours scouring over lab tests for lens sharpness, contrast, distortion, etc. I've probably done that for both these lenses more than once.
But bottom line is, that when I had both in my hands, at least this ONE Tamron beat that ONE Canon lens, hands down.
Left me feeling pretty happy about my purchase, so that's the take-away for me.
Also made me realize that I no longer wonder if I should have bought the 100-400 instead, which is always a good feeling for a guy on a budget.