Hi, Shaun. At this point, video conferencing is tied to XMPP services. So you can turn on/off web conferencing on a per domain basis just like you can currently with group chat.
As for bandwidth, it should be negligible. We're tied into a third party now, so the video portion is hosted through their web app, not your datacenter/SmarterMail server. That said, we DO plan to build out a web conferencing solution, but that's quite the undertaking. SmarterMail 16.x+ will come in stages...there's only so much code we can write and STILL release a product :).