Hello,
I recently tested many of my lens, including a Mamiya AF 55mm, a shift 50mm, a 45mm, two 150mm, and two 80mm, and several Rodenstock Apo Sironar digital lens. My 55mm tested equal or better than all the other Mamiya lens. The shift 50mm lens equals it for center sharpness when not shifted but when shifted the resolution degrades quickly. I have also read other user reviews saying the Mamiya 55mm was one of the sharper lens in their kit.
My one recommendation would be to try focusing manually and using the digital back to check at 100%, if you haven't already done this. Three times out of four, the auto focus on my 55mm does not achieve the sharpest focus, and I need to switch focus to manual and turn the focusing ring a degree at a time until sharpest focus is achieved. Curiously, I almost always turning the ring clock wise to get sharper focus.
Drew