Lots of effort and I appreciate your efforts.
Here are some thoughts, though they come with the very strong admittance that the changes in exposure and the way the camera behaves because of the different image scale/pixel ratio is hard to fully assimilate into my own observations.
First, and perhaps most noticeable to me is that my first pass was not looking at the vignetting so much as it was looking at limiting magnitude and this is what I see. It looked to me like the prime focus view showed much fainter stars than the other views. Now, this is again possibly as much to do with the scale on the chip as anything else, but in the first image, I could see fainter magnitude stars around the target star than I could in the other views.
Second, and something I have been saying for quite some time, is that where illumination falloff takes the real toll is when the nebula is larger than the field of view, and this is quite common when viewing very large nebular regions. This is really well shown in the Gamma Gygni pictures where the field is probably fully illuminated over most of the photocatode. The illumination falloff of a compressed image is going to mean that one trades some brightness for the area at the center of the field for seeing less detail and structure at the edge of the field. These last pictures show that very well in that the nebula, particularly near the edge of the field in between 9 and 12 is quite detailed, and getting it by using any kind of aggressive reducer is likely to suppress this structure.
So, it is a two bladed knife. Achieving a wide field with reducers for the purpose of viewing nebula means that you get a wider field, but the loss of illumiation works against showing you the complete range of structure in the true field. You might get more brightness at the center but here, the reduced angular size may prevent one from seeking out the finest detail.
I am not saying one way is better than the other, but with everything in optics and telescopes, there is always a compromise.
This is why I recommend starting with scopes that have by design, a fast focal ratio and a very large, fully illuminated field.
With reflectors, the secondary mirror is almost always the limit and pushing past the design (which is of course based on a 7mm exit pupil and the low light behavior of the human eye that can't easily see the illumination falloff anyway).
With SCTs, the situation is worse, with edge of field illumination falling to miserable levels once reduced past about f/5 but even at .5x of reduction, the field illumination loss at the edge of the field of an SCT (when using an image intensifer) is hard to see.
With refractors, generally the front of the focuser tube imposes the same limit as the central baffle of the SCT. You can only go so wide before the field illumination falls off, but unless the reduction is quite aggressive (and I have to say that in some of these cases, that is likely what is happening) this may not be easily seen. Shortening a refractor to be able to reach focus though does induce the possibility of aperture reduction occurring due to the objective side of the focuser tube intruding into the light path.
This is nice work though. It cleanly shows the vignetting, but it may not really show the true effects of illumination falloff on extended targets.
My request/recommendation would be this because (in my opinion) the most powerful driver of focal ratio is nebula. Try finding a small, dim extended target that fits easily into the size true field you estimate would be fully illuminated. Take a picture of the target there, and then move the brightest center of that target to the edge of the field and image it again. This would be something I would enjoy seeing and might be useful to others in trying to decide whether aggressive reduction would be useful to them when exploring larger nebula.
I think afocal is probably the most effective way to go (if the scope is not an astrograph) because any good designer would have allowed for about a minimum of a 10mm fully illuminated field and at least 75% illumination at the edge of the field when using the largest field stop that would work with a 7mm exit pupil.
Again, this is why I recommend astrographs. They are designed to provide near full illumination over the full frame of a given size chip. My Boren Simon delivers very high illumination over the size of an APS-C sensor, and this is close to the size of the photocathode. I am very pleased with the way nebula remains as bright at the edge of the field as it is at the center and this is why I request/recommend this kind of comparison. II don't think is really not all about true field for nebula, but rather finding a balance that shows you the most nebula possible in any given field size.
Wonderful work though and it very dramatically illustrates the vignetting, but I am perplexed as to why the prime focus image shows the best limiting magnitude on stars. I could see several stars (going by position) in the prime focus view that were difficult to see in the other views. Maybe the compression used caused this???