I have completed some testing shooting on the Lagoon Nebula, and processed it using the following pallet: Ha (red),OIII (green), and SII (blue). The light subs (10*360" = one hour of signal for each filter) were, I believe, actually over-exposed to a certain degree, not something usually an issue with narrow band from what I understand. I guess shooting at f5.3 for six minutes each at ISO1600 was just too much exposure for such a bright object. I will reshoot in the future with shorter subs (at least for Ha and OIII), and/or lower ISO, to try and stop the burn out in the centre.
So here is the Lagoon image as it sits on Astrobin:
http://www.astrobin.com/full/245563/0/
I believe the resolution is suffering due to the curve and exposure manipulation I had to do to reduce the brightness in the centre. Individual unprocessed subs in Ha look much better in terms of resolution. More to learn for sure.
Note I have deleted the previous Eta C test file (from the post below this one) as it frankly was embarrasing.