My tests show that CHDK impact is about 5-7% battery life or less.
Regarding to S95/S100 and CHDK.
CHDK provide some useful features even for such advanced camera. I found to be useful AutoISO control, badpixel filter, scripting and autofocus/zoom during video.
On one hand no CHDK available for S100 so no such feature could be used (but I believe that camera still cover most requests even without any extension).
In other hand S100 have better optic, video resolution and zooming is silent operation (no motor sound). This is couldn't be emulated by CHDK.
What is more important for you is your choice.