I own that same lens (which I'm selling--I just don't use it) and I've noticed the same problem. I couldn't tell you why, but it's only when I'm at full wide that it's a problem. I usually just zoom in until it's gone, and move the camera back to re-frame the shot if necessary.
unfortunately, its just the nature of a super wide angle lense. you can only bend the light so much before you start getting some sort of issues. youll notice that even if you just zoom all the way out with the standard DVX lense, its can be a little softer on the edges. the light through the edge of the lense is being bent the most, and has the hardest time hitting the sensors with integrity.
The Century .7 is known to behave that way. It's sharp at full telephoto, and soft at full wide angle. The 16x9inc .8x works the opposite: it's razor sharp at full wide angle, but gets really soft and mushy in the corners at full telephoto.
I think the Century .7 is probably at the bottom of the list of their lens converter lineup.