It's tricky, but I have seen a difference, for example in the exif metadata. However, I have had trouble with this, even between successive shots. One possibliity is to check the parameters of CHDK related to autoiso.
I have consternated and experimented numerous times. Several settings seem to interact in ways that are not obvious to me. For example, just setting over- and under- exposure seems to make a difference in some configurations; while at the same time, it affects the brightness of the LCD.
If there is a lucid account of how to set these parameters, I would also appreciate to learn more about it.