Al,
with my 120mm refractor + various spacers/FR's + DSI3 I can get down to about 5 ang/px.
Trying to go lower, results in focus becomes an issue as the "fishtails" at either end of the spectra are getting quite severe, almost to the point of having to focus at different areas and then "stitch" them together later.
I can get down to 2 ang/px by placing the SA directly in front of the camera lens on the 350D, working with a 30mm aperture and 270mm lens gives approx 2 ang/px. It's good for the brighter objects at least
Mark