Hi Eric...What I have read about the 40D it sounds like a great astro camera.
Having 14bit color instead of 12bit for the older models as well as the live focus. I have heard the live focus works ok on bright stars but not much good on anything fainter than 5-6 mag (depending on your scope)?
Not sure on the noise levels but I think they are good at the slower iso settings and maybe a tad higher at the faster isos than the 20D.
You also have no amp glow to worry about.
There is a filter called the x-nite CC1 filter that has a very close transmission curves to the older canon internal filters which should work perfectly for daytime shooting with a modded camera..I am not sure how well this would work with the 40D but as I have no idea of what the transmission of its internal filter? but would be very close I say..
Don`t understand the banding but? that should have nothing to do with filter modification or removal..could be the external power supply that this chap was using or a fault with the camera? I don`t think that the sensor cleaning would be active after the mod either?
But all in all I would love to upgrade to this camera myself...
Sorry I`m not much more help...
cheers Gary