wow, so much info on this camera! Thanks Ray.
With your suggestion in mind, I took a single 20min sub in Ha with gain at 0 the other night as I was packing up. Turned out as the sharpest pillars I've captured to date... at a 80mm and 450mm focal length, its attached below, cropped like crazy. I think 20mins is too long, even for 0 gain, but I think there is certainly plenty if life below 139.
As an indication of my laziness, re-doing my darks is actually holding me back from making the change across the board