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Interesting range of experiences, ive only used 1600. My point was I guess, that in the end any way to increase signal, also increases noise, its just which way gives the best S/N ratio. Lower ISOs need longer exposures (or more subs) or more stretching, but high ISOs are noisier because of higher amp gain. It seems perhaps it depends on the cam, and object brightness (or Houghys cooled cam, no brainer ;-).
I still think that for the dimmest objects 1600 overall would be necessary, especially if exposures need to get much past 5 mins at lower ISOs, given thermal noise would be getting overwhelming.
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