I have a 500mm f/4 L and until recently used it with my 30d. The results are just as good as those taken with my new body (1d mk II n) so don't worry about using it with a 40d.
Results with a 1.4x are also very good with hardly any noticable lack of quality.
There are plenty of examples in my
Gallery if you want to compare. I almost always put the camera/lens/converter info in the camera info section.
BillyPilgrim Gallery - Wildlife Photography