Sigma has published performance data for its 24-35mm f/2 DG HSM Art (pre-order). Check it out below.
I also included an MTF comparison between the f/2 zoom lens and the 24mm and 35mm f/1.4 Art.
Diffraction MTF Charts
Geometrical MTF Charts
MTF versus 24mm f/1.4 Art and 35mm f/1.4 Art
Be aware that this is wide open performance, so the 24-35mm Art is at f/2 and the two prime lenses are at f/1.4. Regrettably, Sigma doesn’t provide MTF charts for stopped down apertures.
Moreover, the diffraction MTF chart for the 35mm f/1.4 Art is not available, so what you see below is the “geometrical” one, that shows very slightly higher (better) values.