Help with a Tokina 11-16

I recently decided to purchase a Tokina 11-16 for an upcoming trip and am trying to figure out if the lens functions properly. I've taken several pictures with mixed results but mostly I'm seeing real softness in the corners. Some of the images seem to be softer on the left side than the right.

Now let me say this up front. I am new to UWA and realize it can be tricky to use so my guess is that these issues may be user error.

Today, I attempted to do an unscientific test by shooting a block wall. I shot it at F 2.8 and F 11, mirror up on a tripod using self timer. The results are as follows....

F 2.8

F 11

Do you see anything wrong with the results here? My opinion is that the 2.8 shot is extremely soft in the corners while the F 11 is better although not as sharp as I had hoped after reading the glowing reviews of the lens.

I can also post other images where the lens appears softer on the left side than the right if interested however after viewing these results I feel it may be user error on my part.

Are soft corners pretty much a given on UWA lenses? Is this a bad copy? Should I send the lens to Tokina for repair?

Thank you for reading and your opinions and advice would be greatly appreciated!--
http://gregp.smugmug.com/