Just received my C300. Here's a very quick test I did with a selection of low light shots, most were around 3200 iso. I was on my own so reluctantly I had to appear in frame.
Lens used -
Zeiss Planar 50mm F1.4
Canon L 24-70mm
I had to add in some grain due to banding on the h.264 encode, first shot in particular was bad, however the native footage was clean.