Here's a good test I just did with my Tokina 12-24. Like most, mine is very sharp, especially at 12mm. In fact, there's virtually no difference at 12mm between f/4 and f/11. It's very sharp across the entire frame. At 24mm, it's still tack sharp in the center at f/4, but it's a bit soft at the extreme corner. (these crops are the very upper corner..and it's at this point it goes soft...just a little bit in is just as sharp as the center). All of these are 100% crops, with very mild sharpening in RAW conversion (34%, with no additional USM in photoshop).
They are kinda compressed to make uploading faster, but one can still see what the lens is capable of. Bear in mind that the shots were taken at the maximum aperture.
