[Wavytone]

Ben, An awful lot depends on the actual scope, the magnification - and what you're looking at. Seeing is pretty much irrelevant below 150X, and on extended soft objects like galaxies or nebulae. Where it really counts is lunar & planetary and this is where the question of the actual scope also becomes important.

Recently Alex & I spent a night comparing planetary eyepieces using his SCT and my Santel, but something else was quite remarkable - at the same magnification the seeing in the SCT was very much worse than in the Santel. His Meade SCT (20cm, f/10 and one with good optics, by SCT standards) showed a remarkable vulnerability to seeing, whereas the Santel (228mm, f/13 and strehl 96.5%) showed images that shook a bit like jelly, but the details in the image remained evident.

FWIW:

a) the Santel easily showed the disks of the galilean moons - despite the seeing - whereas the SCT was really struggling to show them as better than points,
b) on Jupiter the SCT showed details when the seeing settled, but 50% of the time the details were smeared. In comparison the Santel showed details on Jupiter consistently and clearly, though wobbling like a jelly.
c) the Santel showed Saturn with the crepe ring and a retinue of little moons that were simply beyond the grasp of the SCT.

However the Santel took an hour to cool down to kill its internal thermal plume (it doesn't have a fan - yet), meanwhile the SCT was battling dew.

One aspect of a very high quality objective is that they seem to be less affected by poor seeing than a mediocre objective, and this appears to be true for refractors as well as reflectors - including Newtonians. An objective that puts ALL the photons exactly where they should be - bar none - has a distinct advantage over one that doesn't when looking at objects that are marginal - either faint stars or moons (Saturn) or on resolution.

In particular it is one of the reasons why a triplet APO performs FAR better than an achro doublet. It is also why some buy Questars - their little 90mm Maks are legendary. And why someone will shell out 12,000 euros for a 25cm Mak from Matthias Wirth - not a C11 or C14.

I'd be interested to see someone compare two 20cm dobs side by side at around 200X magnification in average seeing in planetary targets - one with superlative optics and one with average optics. My guess is the good one will be less affected by seeing than the average one.