Many thanks. Yes even the unprocessed image as seen on the camera LCD after I took them showed far more nebulosity than shots without a filter. The trick for these long high ISO exposures is, converting the Raw images to Tiffs using Photoshop, rather than the supplied Canon software. The Photoshop RAW plugin somehow seems to get rid of virtually all dark current nouse except the amp glow on right side of shots.
I can only wonder how well it would do if I was at a dark site, had a nice recoated mirror, and had the modified DSLR (IR cut filter replaced)