One of the first lenses I got for the Sony FE System was the 28mm f2.0, which is very sharp stopped down, but still small and light. As 28 mm was a bit narrow for some of my use around Laufenburg, I got as well the wide-angle converter SEL 075-UWC, which converts the 28mm f2 to a 21mm f2.8. This converter was at first glance quite practical, and handling was top-notch, with proper communication of the lens with the camera regarding whether the adapter was mounted (through a little magnet), resulting in correct EXIF values for focal length and aperture and proper settings for the in-body image stabilisation (IBIS).
After all I was not happy with the results of the converter, so I tested wether the Tokina 20mm f2.0 MF would provide better image quality. I made an aperture series with the camera on a tripod and 2s self-release using both lenses throughout their aperture range and provide crops from center, mid-frame and corner to compare the sharpness in those regions of the frame.
The results were quite clear, the wide-angle converter is never as sharp in the corners as the Tokina lens and very soft at lower (less than 5.6) aperture values. I subsequently decided to sell the converter and keep the Tokina for my landscape/cityscape photographs.
Even the bokeh is quite smooth, although you have to be very close to your subject to get some.