Latest article by Craig Stark on long exposures,
see CN here. Shows the gain given for xsi and more. Craig has done a lot of articles on DSLRs and how to measure your individual camera performance,
here.
However, basically the conclusion is that Canon is re-scaling image data before writing image files. The most interesting point is that ISO400 may be the best settings after all with wider dynamic range and less image scaling due to lower noise.
Anyway...FWIW.