I owned and shot the Mamiya version of the 28mm for about 2 1/2 years. My results are similar to yours, sweet spot is F11 to F16. Your corners are actually a bit better than mine were. As Guy mentioned, the ability to sharpen the corners in Capture one do help a lot on this lens. Mine had a bit more detail smearing especially when used around F5 to F8.
For the the 28mm was not very filter friendly. I used a home made rig to allow me to use a Cokin X-pro holder over the built in shade. I believe Phase One now offers a similar feature for this lens, I remember seeing it on the web during Photokina.
Doug's testing may prove me wrong, but I also don't think that there was much optical improvement in the Leaf version of the lens.
I switched over to the Rodenstock 28mm HR and have found it provides a much more enjoyable shooting experience, especially with the addition of tilt.
I also used the 35mm Mamiya for about the same time or longer. Mine was a bit better in the corners when I was using a P45+, however that sensor is a 1:1 crop. When I started using the 35mm on a 60mp sensor I quickly found that it was having much more trouble resolving to the sensor. Mine was a older lens and I briefly tried a new one from B&H but found it preformed about the same.
My single biggest complaint was the fact that neither of these lens were very good wide open, or even stopped down to say F6.3 or 7. Shooting conditions I work in don't always allow for F11 to F16. Also when I was using my P45+ on night shoots, I found the 35mm much better than the 28mm since my 28mm in the wider apertures showed considerable detail smearing. Softness might be recoverable smearing can't be.