Vignetting is the usual price you have to pay if you are buying a small lightweight lens.
According to Bryan’s site, you don’t even really get rid of vignetting by stopping down the lens to f/8. It seems to penetrate visibly rather deep in the frame. But is it a real problem? In other respects the lens seems to be fine and small in its class. If you’re shooting landscapes on tripod or in reasonable good light hand held and stop down the lens at least a little bit, you can always remove vignetting without doing too much damage to IQ. In the corners where the vignetting is at its worst, the IQ won’t be that tasty anyway and it usually isn’t the place where you want to guide viewer’s eyes in the first place.
BTW, dxo is giving us very different numbers. Maybe they are using autocorrected jpegs…
I am shooting with Sigma 20/1.4 Art (with Nikon D600). It is a bulky (close to 1 kg) lens, but handles well in a full-frame DSLR. I wouldn’t buy it for landscape shooting, because IQ close to borders isn’t any good at wide apertures and it has issues with field curvature => you have to stop it down and IMHO it doesn’t make much sense to buy a bulky fast prime and use it stopped down when you can do the same and much more with a more versatile zoom. Other problems are coma, which is rather severe wide open, and performance against bright light could be better, especially when the lens is stopped down.
I bought it for portraiture, event photography and creative low light shooting and for these purposes it is a good lense. IQ in central region is good wide open and “look” and bokeh are nice.