I think the answer to your question is probably yes, but how much you would notice and is this what you are seeing is another question. I have had the same question and thought the same thing with really long lenses with narrow DOF.

Another thought;

iND I think if you look at the ISO charts you might figure the answer out on this one. You will notice in the corners that the lens falls off and will be sharp in the center. It seems that newer lenses that Canon has released the corners have improved and the center of the lens has not in comparison. Perhaps this is what you are seeing in your pictures?