My own take on using wide RF lenses on the A7r: not worth the bother. You can dial out magenta contamination at the left & right sides of the frame and perhaps even improve corner clarity via deconvolution techniques, but I'd rather just work with cleaner data. Of course this means larger SLR wide lenses. As I'm not an ultra-wide kinda guy I can manage fine with a 24–28mm f/2.8...these aren't too bulky or heavy.
Now this is just me. YMMV.
-Dave-