Recently, i bought one from Guy (Thanks Guy! ) . This is custom modified to fit the Sony RX1R II (which is still at the repair shop).
While it works well with wider lenses like the Sony FE 28/2 it was made for and Sone E 16/20 (same bayonet connection), there is something interesting mismatch on the longer FE 55/1.8.
On this lens, the UWC, besides making it wider also projects fuzzy, dreamy images with a glow from f/1.8 to f/2.5. This effect is kept at all focus distances all the way to infinity. It is gone by f/4 though.
Because of the fuzz, AF does not work well and manual focus is mandatory.
I will post some samples when I get some more time.