I had a quick look at the FITS file output from a Nebulosity stack, saved it as 16 bit TIF, and opened it in PS setting working colour space to ProPhoto RGB. After just a little bit of stretching there was a noticeable difference. Think I'll be adopting Humayun's workflow now that I (think I) understand the benefits. The ProPhoto RGB colour space seems to be the closest fit to the colour space that my camera's sensor captures based on some of the reading I've recently done.
Check out
this article from Luminous Landscape. See the rollover image showing the ProPhoto RGB compared to 20D sensor and the better coverage in that front corner of the reds? I instantly saw that come out in my image test. If I get a chance I'll post the comparison. Much richer colours, not as washed out.
Putting this colour management together with some other processing tips and tricks I'm learning at the moment and seeing vast improvement. Looking forward now to new data, fully calibrated and colour managed!