How an S100 compares to a 600D at high iso? You're te judge! (Warning: jpeg only!)
The 600D is better, of course, but by much less than I had thought actually. About two stops at low ISO and 1,5 stop or less at high iso.
If I understood your original post, you seem surprised that the relative advantage of the 600D decreases with higher ISO. That shouldn't be surprising... noise isn't linear with ISO setting.
I'm not too surpised that there's only a 2-stop advantage for your test conditions. P&S cameras are much improved over a few years ago (I still remember cringing at the thought of using ISO800 on my S3is, and never did go beyond 400). Also, you didn't get the best performance out of your 600D, shooting in jpeg and not raw.