1. You can overcome read noise with subs that are long enough regardless of camera read noise
2. Shot noise is unavoidable and is sqrt(n) where n is the number of photons you detected (per pixel)
That's really all you need to know, Dunk
You use sky limited subs and you stop when the dimmest parts of the image that you are interested in look acceptably clean. Noise reduction is acceptable, IMO, when selectively applied so that data that already has acceptable SNR is not adversely affected.
The only time you get SNR that is consistent over a whole image is when you have a very bright source, like when you take flats. With the objects we normally image there will be areas of good SNR and areas of poor SNR. If you want to see the dimmest areas looking clean you'll need to get a very large amount of integration time under dark skies. A short amount of integration time under very bright skies will also get you high SNR, but this is only useful if you like imaging light pollution gradients
