As Justin mentioned I've done a bit of experimenting with different sub durations and ISO settings. My approach has basically been to determine the settings which will maximise the signal:noise ratio. see
here for an example of how I have applied this.
If you find that the ideal SNR settings for a given ambient temperature result in too much loss of star colour, one strategy could be to take a second set of subs at a shorter sub length and then do a HDR combine.