Carl,
I think you need to consider what is happening when you change these filters....
1. The original filter in the camera is designed to suppress the UV and IR to give a "realistic" natural colouring with minimal chromatic focus issues. Unfortunately for astro imaging the cut-off wavelength in the IR is close to the Ha emission and the efficiency in this area is pretty low (around 25%)
2. If the filter is replaced with a Baader filter (or similar) then the cut-off wavelengths are much more defined (400nm and 700nm) - this allows a much higher efficiency at Ha (up to 50-60%). Yoy still have the benefit of the UV-IR suppression to reduce star image "bloating".
3. If you remove the filter completely...then all the UV and IR light is allowed through. Great for spectroscopy
but it does mean in refractors that the star images will look bloated due to the excess UV (blue ) or the NIR (red). You also need to use the inbuilt Custom colour balance to use the camera for "normal family shots"
4. For option 4 you can "compensate" by adding a clip in filter which effective operates like option#3 Some filters, besides providing UV-IR cut-off can also act as a light pollution filter (ie CLS etc)
In all the modern Canon cameras (400D onwards) there are two filters - one is a "mild" UV-IR which has the shake rattle an' roll for sensor cleaning and a second filter (much thinner than the original back in the 300/ 350D series) which is the actual "heavy" UV-IR filter - this is the one that is normally replaced/ removed. As it's so thin, it can still be removed and the camera still focus normally with standard lenses ( which was NOT the case with the 300/ 350D)
HTH