I have just done a quick test for you, hand-held, with the 20mm Laowa lens on a GFX 100 and the results are excellent, although I have not yet seen them on a computer monitor, although substantially zoomed on the camera back.
I set the aperture at f/8, which would with most lenses been a good aperture to use for most photography, and fully shifted left along the long axis, with the camera horizontal for one shot and then fully shifted right for the next shot. I have observed very minor light loss (image darkening) at the edge opposite the direction of shift, apparently less than with the 15mm lens, and easily correctable in imaging software.
One must double-check the orientation of the lens shade, which is rotatable, to avoid vignetting from it, and where possible I think that I would remove the lens shade when shooting fully shifted, just in case it slightly shaded the edge of the image area.
Hope this is helpful.
For landscape photography I sometimes like to shift fully left and then fully right and to stitch the resulting images on the computer, and I have done this with the 15mm Laowa lens on the Sony "full frame" camera, but not yet with the 20mm lens on the GFX. Again, where possible this is best done using a tripod, but I have also obtained some excellent results with the camera hand-held, doing my best to avoid changing the height of the image in the frame.