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Paul, there is certainly a description of seeing which defines the average size of seeing cells in the atmosphere which will produce diffraction limited wavefront with perhaps only tilt variation that can be taken out with a tip tilt adaptive optics device ( in other words a stable airy disc which is moving about )
A long term study using interferometry by the Sydney University Stellar Interferometer found diffraction limited seeing at 150mm aperture at Lindfield on an acceptable number of nights and at Narrabri closer to 200mm.
There are some good academic papers around on 'seeing' . I'll have to see if I can locate some online.
Your post suggests that thermal management is all that is required to get superb views out of a large instrument. This is an oversimplification. Bad seeing will limit large scopes aesthetically on most nights. If you are happy to accept fleeting glimpses of fine detail with a large scope ( like I am ) or be prepared to stack your good frames ( like you do ) and manage thermal issues , then large scope will always be more useful than smaller ones.
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