An interesting comparison, Ray.
A few quick thoughts:
With option 3 are you assuming an improvement in read noise from hardware binning? If you're sky limited then software binning should work nearly as well and you'd be able to add the RGB data in to luminance.
Have you thought about the impact of noise reduction? It seems to me that binning is really a dumb form of NR. Dumb in the sense that it blurs all the data, not just the low SNR areas.
I have used option 4 with bright, colourful targets, like M31, and it works brilliantly in that situation.
Wrt Drizzling, it does complicated things with noise (see
http://www.stsci.edu/hst/HST_overvie...34.html#385457 if you're interested). In practice, I think it's effectively a noise reduction technique as the correlated noise is less obvious visually.
Cheers,
Rick.