It's not an adapter. They change the R mount to an Alpha mount with the Sony CPU chip so it meters and enables SSS stabilization.Sorry Dave, I tough you didn't, that is an Italian reading here
I will definitely consider a Fisheye, but only if I can't find a fit to my needs in either a Prime, or zoom for it, I really like to stay with original Alpha mount lenses and avoid any kind of adapters, asI visited Marc's link and even so I love Leica glass, I just don't like fittings and any type of adapters, they never really work out for me, one way or an other.
Nothing in the Sony line-up is going to equal the latest Leica R 19/2.8 for the IQ requirement you outlined in your initial post.
If it's Sony or nothing, then I'd say go for the 16-35.
If your luck is anything like mine, the minute you do go for the 16-35, Sony will announce the legendary Zeiss 21/2.8 Distagon is now available in AF ZA mount ...:ROTFL: (I wish.)
-Marc