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Richard your data is interesting. I must try this out. My only concern is that SNR does not necessarily equate to a properly exposed image or does it? At iso800, 5 mins,under my sky, detail is obliterated and the image over exposed/bright. 3 - 3.5 is much better. Signal is signal, whether overdone or not, it contributes to SNR. Dynamic range is also a consideration and is inversely proportional to iso - is it significant between 400 and 1600? Does it matter? Are slower integration times better under polluted skies?
A fix for over exposed areas is combining shorter subs, which means adding data with significantly lower SNR - M42 trapezium.
Dithering is essential with DSLRs.
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