It is a common misconception that seeing somehow hides optical imperfections and you can't get benefit from good optics if the seeing is average. In reality, first the seeing distorts the wavefront at some amount and then the telescopes optical aberrations add on to that and smudge the already smudged image even further. So better optics give always better image, no matter of seeing, if other factors (aperture etc.) are kept the same. And when we go to high-res and especially lucky imaging, the quality of the optics is even more important in less than ideal seeing: Seeing is a somewhat statistical phenomenon and even in poor seeing, there is always some probability for good frames and if you capture 50000 frames of Jupiter, there is maybe still 300 good frames and you get an image. But if your optics are bad, even those 300 good frames become bad and you don't get an image.
The above post is an exception to the rule (IMO). I find even a mass produced 12" is more pleasing than a 10" Premium for DSO's.
For Planetary/Lunar It's trickier but the premium mirror needs excellent seeing to overcome even the 2" Aperture difference.
Maybe in general, but the OP was specifically asking about optical quality and resolution on 20" Stargate Dob, so it is not an exeption unfortunately. There are at least two threads in this forum on 20" stargate and maybe one on 18" stargate, and my case is not the worst at all. For example see this post from one of those threads: https://www.cloudyni...vice/?p=9484986
If one wants to do high resolution work, Stargate 20" is not the preferred choice. Much better odds with the 16":er.
Ironically, after 2 lousy SW20" mirrors, I donated the optics away and modified my Stargate for 24" custom made mirror that turned out to be bad as well... The manufacturer refigured it once, but we still couldn't get it to work, so took it back. Now, after 4 years since I originally bought the Stargate, I have this huge 24" Stargate based scope laying around without optics
In two months or so, I will receive 24" Mirrosphere (Franck Griere) optics for it. I will then repeat the side by side comparison for my SW16", with and without masking the aperture to the same and even smaller, maybe on several nights to have different seeing, to get some actual images on the "premium effect".