each exchanged key = 16 bytes if i'm not mistaken
so lets say you view a channel for 1 hour, if the key is updated every 5 seconds, that 12 exchanges / minute => 11520 bytes transferred to send or recieve the keys (that is 0.01 Megabytes) ...
so all this to say that i thikn you are quite safe to not be worried about the bandwith used by CS
pls someone correct me if i'm misconcepted about something
cheers