Let's dispel the theory the Mamiya 28 or most likely the Hassy 28mm is soft in the corners. I shoot the Mamiya 28mm a lot and yes it is soft in the corners when you are very close to a subject and mostly it is the curvature of the lens and nothing more. These are ALL shot with a 28mm lens and every corner is extremely good. Let's also say there are ONLY 2 28mm around the Hassy and the mamiya and ANY solution outside of these two you can get out your ski mask and need to go rob a bank. Any other solution starts at 6k or more( Rodenstock 28mm is 6k alone) and up to 16k. Worst case use it as a 30mm lens when you are close to a subject but as you can see in these the minimum distance is STILL 5ft and there sharp.
Guy, I love you like a Bro, you know that, but I am on my third 28D and though it is very good compared to the first two, it does have soft corners at anything below F16 - and the best on-centre results are between F8 and F11.
To be more precise, part of the problem is softness and part of it is DOF: in other words, even on a man-height tripod, the immediate foreground is too close for great resolution unless you stop down to F16. However if you shoot off the edge of a cliff or escarpment, so that the immediate bottom of frame subject is further away, things improve radically.
Your files look great but they are 800 pixel web shots and even you yourself have posted the famous 'Mancuso Method' for tricking C1 into sharpening the corners of 28D files! Also, the work you do is much more suited to focus stacking than is mine.
This all brings up the question of what is the sensor-size 'sweet spot' for each individual. For me, for example, if you could get infinite resolution onto chips of any size, I'd probably go for something like the M8 sensor size: you can get good wides to (35mm equiv) about 20mm FOV, go as long as you like and have a good choice of 'everything is in focus' glass and 'selective DOF with gorgeous bokeh' glass...
