We’re talking 5000+ users, not a few hundreds. A community that has to constantly take attacks from 4chan and other vile places of the internet. I do not believe IRC would be the tool for managing this community not only coherently but as a unit.

Mumble solves audio chat, but not screencasting, which we often have in the game related channels. Audio chats routinely hold more than 20 users, with one or two people screencasting at 1080p30. Even if we used mumble, it would split the community from IRC as the people on Mumble can’t coherently interact with IRC in the same client.