Sigma 30/2.8 DN is not particularly well suited for macro photography because of its dismal max magnification of 0.12x at the MFD. The lens suffers a little bit in sharpness at its widest aperture, but only a little bit. With the EFOV of 45mm, the lens turns into a normal-ish prime, so it's would not be out of question to shoot it at the MFD to get the shallowest DOF
Sigma 30/2.8 EX DN showed moderate degree of coma around corners at f/2.8, which is reduced with stopped down apertures. By f/8 it is very minimal to non-existent. The crops below demonstrate what you can expect with the lens - images were taken at ~1.5m focusing distance and 3s exposure time.