Many valid points have been raised, and surely a Riccardi-Honders is great in practice with its large fully corrected field, but isn't that design mostly of benefit in wide field applications?
I found some numbers to shed a bit more light on my original question:
Median FWHM of Paranal site: 0.75"
Median FWHM of Kitt Peak site: 1.10"
Airy disk size for 8000mm (VLT) aperture: 0.03"
Airy disk size for 250mm aperture: 1.03" = 6.2microns @ f/5
So the conclusion from that must be that atmospheric seeing is by far the limiting factor for a site like VLT, effectively reducing the theoretical resolution by a factor of 25.
For my location, which isn't on a remote mountain top with premium seeing conditions, I'd expect the average achievable FWHM to then be around maybe 2.00". I'm just guessing here, but it must surely be worse than the professional sites. So that means in my case the atmospheric seeing reduces my theoretical resolution by a factor of ~2. The question then is wether a GSO 250mm mirror is good enough to deliver star sizes no larger than the average seeing of 2.00"? and based on my experience I think it would be good enough, since with my old no-name mirror I can see much greater detail that this visually when looking at Jupiter for example.
So simply in the context of 'which f/5 mirror to use for long exposure deep sky inaging', I think a GSO would more than suffice based on what I can gather here.
In any case this is a very interesting discussion overall, keep it coming!
