Hi,
I am not sure if i am reading the graph correctly but the following suggests that an unmodded 40D has a much worse response to Ha than an unmodded 350D.
Less than 10% Ha is let through? If this is true then its like shooting at F50 for Ha so I would agree with bassnut and say mod it.
http://www.astrosurf.org/buil/eos40d2/filter.htm
Paul
Edit.. Actually the graph is relative response but i still reckon mod it
