Gary, I think your soup can photos indicate a user error and not a problem with the lens. I could be wrong, and don't get mad, I'm just trying to help (this coming from an amateur).
The Sigma 30mm f/1.4 EX DC at 1.4, as it is in the first photo, has such a shallow DOF that certain things in the frame may be more out of focus. Look at my post above your post with all of the numbers in it. Because of the shallow DOF the can on the right is more in focus than the can in the middle? Why? It is closer to your lens. Look at the curvature of the can. The further away the sides of the cans are, the less in focus they are. It is obvious that the can on the right is closer to your camera, and thus more in focus. Predictably, in photo # 2, using an aperture setting of 2.0 the cans are clearer. That is to be expected. If I'm all wet on this analysis please tell me.
Like I said above, you really need to know what you want to focus on when using an extremely low aperture setting. Failing to do so will result is some blur, which can be cool in some circumstances and not-so-cool in other cases. But this phenomenon occurs with any lens. It is only accentuated with such a fast lens. With a super, super fast lens like the Leica 50mm f/1.0 Noctilux-M, the DOF is even shallower and even more tricky. Talk to anyone who has used a Noctilux and I think they will tell you it is an incredibly tricky lens to use. But the bokeh on it is incredible. That's a similar trade-off you have with the Sigma 30mm f/1.4 EX DC. And your assumption that the lens is faulty and your complaint is similar to those other people who have given up on this lens. You need to use it much more before you call it a faulty lens. Use the Sigma 30mm f/1.4 EX DC as your primary lens for two months, I am told, and you will learn more about photography than anyone can ever teach you.
Well, won't get mad but in your photo the object you focused on, the middle one is nice and sharp right?
Well I focused on the middle can which is set back from the other two cans, camera said it achieved focus but in reality the focus was more in front of where I had focused, so that's called front focusing, the second shot shows it stopped down a little and now that can in the middle is some what sharp, but not truely sharp, again very carefully setup on the tripod, mirror lockup, self timer to prevent any intervention of me, and when I shot the same setup with my other lens, the 17-40 set at about 30mm the center can is perfectly focused, so, user error, doesn't appear to me that way, I've shot a lot of shots with other cameras/lens and when a lens provides me out of focus images repeatedly I don't think it's user error in that case. I understand the slim bit at f1.4 and would watch for that but, at the point of focus it still should be sharp, not that far in front.
I'll hopefully get a lens back calibrated that works great and will post what happens, again, when I talked with the tech, I could almost hear him nodding his head as he said just send it, no problem, sounded like they have had this issue before and can easily correct it.
edited to add, that Johnny you pointed out that the cans on the sides are closer to me, if they were not, you would not see any front focusing as there would be nothing to see, so that is why they were moved toward the camera in order to see the area where the real focusing was taking place.
If the lens was Back focusing, the sides of the middle can would have shown in focus.
hope that explains what the images were trying to represent...