So unless there is an error in their measurments the old Tamron should be sharper across the focal length than the Canon L....... and you believe this?
Good question. I have no hands-on experience with the Canon 28-300 L.
I do, however have experience with the old Tamron 28-300 and my personal experience is this:
Stopped down to f/8 it produces images in the 35-200mm range which I find useable.
It has a lot of CA and fringing, but since I only shoot in RAW I have been able to correct most of it in
my raw converter.
At 28mm the corners get soft - but it is subjectively still better than my 7d/17-85mm combo at 17mm.
I previously owned a Canon 300mm F/4 IS. At face value (No mfa - my camera at the time was a 20d),
it was softer than the Tamron @300mm. After purchasing a 5dII - and doing the MFA - it was only slightly sharper wide open than the Tamron.
My point is this: Canon's "L" designation is no guarantee of optical excellence, there is nothing mythical about it.
I'm sure the build quality of the Canon 28-300 L surely is better (the Tamron's zom ring has gone loose, I have to push/pull the hood to zoom).