I have a quite limited knowledge on spot diagrams (can spot bad ones but struggle to read the difference between good ones), MTF curves and the like. Mostly I can spot bad ones and glean some very basic information, something I am slowly trying to teach myself
What I thought I would show is two images that I have taken. Same target, same lens, roughly the same integration time but different cameras.
LMC with Nikon D700 & Nikkor 85mm F/1.8G
Nikon D7200
The first was taken with the Nikon D700 which has 8.445 micron pixels and an anti-aliasing filter which means that I can never get better than a FWHM of ~8 pixels. It was taken wide open at F/1.8 so when I saw a little bit of blue/purple around some stars I really wasn't too surprised. The stars are showing some slight deformity at the edges of the FX frame. Also not too surprised at F/1.8.
The second is with a Nikon D7200 which I think has 3.91 micron pixels and no anti-aliasing filter. It was taken at F/4 so a far cry from wide open. It shows a LOT of purple (the outer star forming regions look purple and not blue) and you can see the stars deforming quite easily in the DX chip. There is however ~3500 galaxies in the PGC catalog in this image!
What these two show is that even an average lens can show almost zero aberrations if quite large pixels are used. Small pixels really show any optical imperfections, especially when small pixels are matched with a higher QE and lower read noise.
In some ways this also points back to what Greg has seen with shorter scopes with small pixels vs larger scopes with larger pixels. On average, most of the shorter focal length telescopes are cheaper and have smaller cheaper cameras and lesser quality correctors so even when you're imaging at the same scale, optical imperfections are more likely to be an issue.
I have recently been looking at short focal length optical nirvana, looking at telescopes like the Tak Epsilon series (F/2.8-3.3) or a Boren-Simon Power Newt 8" F/2.8 (8" F/4 with an ASA 0.73x Corrector/Reducer). These are on the cheaper side for their size and focal ratio and it shows when using cameras like the ASI1600.
Of course then there is the large price increase to the ASA 8" F/2.8 H which has spot diagrams that just make one drool which is effectively what you end up paying for.