Flare control is basically average in this lens. Under the worst lightning conditions, like showcased in the samples below, you should expect a fairly heavy amount of flare, some aperture ghost, color bands and greatly reduced contrast. Sigma did not supply a lens hood with this lens, but it's not like you'd be able to use a lens hood with the lens anyway - the lens does not have a hood bayonet at all. The only option is if you manage to find a 46mm screw-on type lens hood, but I'm not even sure such lens hoods exist.
Sigma 30/2.8 showed some vignetting on NEX-7 - at ~1.5EV with wide open aperture, you would notice darkening of corners in lightly colored scenes. Stopping down to f/4 brings light falloff to ~0.85EV and by f/5.6 vignetting is basically gone.
Camera's built-in shading correction does remove some portion of vignetting, but not completely - still better than nothing. If you choose to correct vignetting in post-processing, you would need to set vignetting correction to +25 for f/2.8.
Sigma 30/2.8 EX DN showed a pretty minimal amount of barrel distortion - at ~0.5% distortion should not be visible under real life situations.