I did this with the lens stopped down to f/5. I have a feeling Mike Sid will say 'I told you so'.
Canon 5DH, Canon 300mm F2.8L at f/5. Hutech LPR filter.
Exposures 10x( 1m, 2m and 4m) at 800ISO. Fridge at -5.0C. The usual hdr method. I have not used any enhancement. I can see a slight improvement with hardly any loss of dim nebulosity.
Full frame 4MB
http://members.optusnet.com.au/~cheekyfish/carf5.jpg
Why I have been reluctant to stop down is that the iris blades cause unwanted diffraction spikes on the very bright stars. I actually used a baffle out of a refractor to stop the lens down by placing it inside the lens hood.
Bert