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This is an interesting discussion Ray. I think you might find that longer subs still (ie 10-15-20 minutes) will remain fairly static with regard to FWHM in similar seeing. The nuances of seeing and guiding will be more obvious from 1 second to 2 minutes in my opinion and then will slowly reduce over increasing sub lengths to where there is little difference. I know from experience very short subs always have tighter star profiles than ones from even 2 minutes. I am sure this boils down to the star moving just minor amounts around the centroid. Even tight guiding in good seeing still sees this happen.
I can see uses for both short and long subs. Bright objects like the homunculus can be imaged with quite short subs. I think I used 1 second subs when I did Eta Carina last year. Other similar objects such as the core of M42 would also similarly respond well to short subs. Both from a saturation level and from a lucky seeing point of view. As you know planetary imaging has been doing this for years. Objects with high brightness levels can be imaged at very fast speeds.
However I can also see the use of longer subs just to pick up signal. I routinely use long subs for detail imaging on both systems. I think loosing a tiny bit of detail sharpness is more than compensated by increase in signal. If imaging from stable seeing locations (I know not everyone has this luxury) some slight blurring can be made up with degrees of sharpening.
Perhaps if sensors can be developed with more sensitivity (though physics will limit how much) than at present, then short subs could become the norm.
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