Here are some test shots from the Sigma 30mm f1,4 and the Canon 28mm f2,8. None of the shots have been sharpened in camera or in Photoshop.
The Sigma is sharper in the center and the Canon is sharper in the corner, when compared at equal apartures.
The center sharpness of the Sigma at f1,4 can be compared with the center sharpness of the Canon at f4. The corner sharpness of the Canon at f4 can be compared to the Sigma at 5,6.