Hey Scott,
Do a dark frame! You'll be amazed at the difference. While your technique works, you can't beat the dark frame. Instead of just averaging away your nasty pixels, you will eliminate them!
Although I appreciate at longer exposure the dark frame is more time consuming!
Be interesting to see the difference in a shot at ISO400 and 8mins exposure time. I wonder which would be better?
I almost always image at ISO1600. Mainly because I'm lazy I think!
...
Cheers