We're using ShareJS in production for the authoring side of http://theconversation.edu.au - it's brilliant and solved most of our collaborative editing pain.
[ShareJS author] I've considered rewriting ShareJS in C a couple times.
For science, I ported the core operational transform algorithm code from coffeescript into C and got a ~10% reduction in code size and an 8x performance boost. The networking and database code won't translate anywhere near as well, but it may still be worth doing at some stage.
If only it wasn't written in node....