Just another view on prices like these.
After testing 2 different 28mm on a P45+, and hearing from other shooters, the lens on this back does have considerable corner softness. It's apparent to me that Mamiya is not testing these new Digital lenses on a 33mp or higher back. Or they are testing them and just willing to put a lens with less than the best optical quality on the market for 5K. 5K to me is a huge investment for one lens, which by the way has no ability to work with any filters. Filters for me, especially a polarizer are critical. I don't count a gel holder a usable filter.
They are charging a huge amount for these lenses and as of yet not able to deliver the quality they claim. They also have nothing larger than a ZD back at their Service center and both 28mm's I had showed corner softness on the ZD also.
My view is if you are going to charge these prices, there really shouldn't be much error.
Paul C