You have to think of it in terms of the shape of the light cone as it passes through the eyepiece. Let's take an extreme example of a custom 8" F3 reflector versus an 8" F20 Mak Cass.
With the F3, you have 8" of aperture that focuses down to a point over a distance of just 24". With the Mak, you have 8" of aperture that focuses down to a point over 160". It's a very different shape. With F3, the size of the light cone is crashing down fast, so it really kind of looks triangular. The F20 is more like a straight line that eventually closes down to a point. As the light passes through the eyepiece, it is easier to handle light traveling in a nearly straight line than it is light coming in at a steep angle. It takes more elements to correct across the field when the light is coming in at a steep angle, versus relatively straight. Now you are at F5.6 not F3, but that's still pretty fast. The Masuyama would resemble Han Solo jumping to hyperspace, with the center of the view being clear and sharp, and the stars around the edge being more like streaks.
Something like the 12.5 HiFW or 12.5 Morpheus will probably have eight elements instead of five. Consequently, there will be a touch more light scatter, but this is just a subtle difference. While the outer field will be much sharper, perhaps even sharp up to the field stop. So you sacrifice a touch of contrast in the center in order to have good correction across the whole view, and be able to use the whole view.
Really, at F5.6, the outer view is going to be so abberated in the Masuyama, that you might as well just cut it off. A 53 Masuyama will deliver the same contrast without the streaky stars around the edge of the view. And it will be cheaper, and perhaps have a bit more eye relief. Do you want 85 AFOV where the central 50 AFOV is sharp and the outer 35 AFOV is messy? Or just 53 AFOV of sharpness?
Or around 80 AFOV of sharpness, but just a touch less sharp in the center?
Those are your choices at F5.6.
