DP1m soft corners
I can't really tell ... I use the DP1M mostly for landscape ... most of my pics are taken at f8. If I want a shallow DOF, chances are that the subject is in the middle or close to the middle, so I am not worried about corner performance that much at f2.8. So I have never done any test to check the corner sharpness at f2.8 ... maybe somebody else can answer this.
I actually used the DP1M just yesterday while hiking and all I can say is that I didn't see a problem with the corners at f8. For a wide angle lens, the DP1M performs very well.