I guess what you are referring to is the heavy noise reduction in the jpeg. AFAIK some cameras give you some control of noise reduction in the stock firmware.
If you are shooting at high ISO, the Canon firmware may use more noise reduction than low ISO.
None of this has anything to do with CHDK, although it is possible that CHDK could be modified to tweak these settings.
If you shoot raw (including DNG), you can do the noise reduction yourself.