Send Sigma an email explaining the problem, maybe they will take it under warranty service.
Yes, I could do that. However, I've already had an unsatisfactory experience doing something similar with Canon equipment a few years ago, so I'm a bit reluctant to risk wasting more of my time.
I was very impressed with the sharpness of my Canon EF-S 17-55/F2.8 zoom, until I discovered that it wouldn't accurately autofocus at close distances when set at F2.8 on my new Canon 40D. I sent both lens and body to Canon service under warranty, to get the problem fixed. They were unable to fix it and claimed that both body and lens were within specification.
The Canon 40D didn't have the AF fine-tune feature, so I was stuck. But within a short period, Canon released an upgrade which did boast the AF fine-tune feature, the 15mp 50D. So I bought it, not just for the AF fine-tune, but that was a major attraction.
The irony is, the 50D focused perfectly at maximum aperture with the EF-S 17-55/2.8, without any AF fine-tune adjustment at all. I can only assume that the AF fine-tune has a dual purpose. To compensate for any slight maladjustment of the lens, and/or to compensate for any slight maladjustment of the camera body.
Both lens and body could be within specifications, that is, within the acceptable margin of error, but there could be situations where the errors in both body and lens are at the same end of the allowable spectrum of error, and have an additive effect.
Conversely, there could be other situations where the errors in both the lens and the body are at opposite ends of the spectrum and cancel each other out, creating the false impression that one is lucky to have an ideal camera body and lens without error.