So...I went ahead and bought the Voigtlander 15mm and thought I'd upload some shots taken on my first outing. They are all shot at f8 with focus set at around 1-1.5 metres. Also, the images are jpegs straight out of the camera.
My first impressions are very positive (with the quality rather than my photographic ability). I can see that there is some corner fall off but I'm hoping that I can fix this in Capture One. With regards to the corner softness, again I can see that it exists, but I'm not sure I'm worried about it at this stage. Anyway I guess time will tell.