Thought this was worth a try. I have the Zeiss 12mm Touit lens for my A6000 which is a 18mm on a Full Frame sensor. So on the A7r I shot it with the APC crop mode on and got the first image a 18mm FOV. It would be the same on the A6000 so no gain here shooting it on the A7r since it would be less than the A6000 24mpx but again it could be useful to the A7r as a 18mm if you are not carrying the A6000 with you or for whatever reason. Its small its really cheap right now and damn good.
But here was more a interest to me on the A7r is with the APC crop mode off your shooting right to the image circle which by default cropped is a square cutting out the vignetting.
So here is what I did I shot normal crop mode than shot it to the image circle processed them both out put a color mask over the APC crop image and laid it perfectly in place over the Image circle image. What you get is a touch of extra room on the sides but depending on your subject you could crop in various directions.
This might actually be useful with interiors if your trying to get more ceiling or floor for that matter or a square may work better. Obviously not limited to interiors but still a quite interesting option if you so want to try it.
This is the APC crop version ala 18mm equiv.
Than this is the full image circle image with the crop overlaid on it so you can see the extra room you may want.