The topical bit is important- a particular app's notifications get dumped into that app's room, for example. That means I only get notified if I'm in that room. Conversely, that means I don't get excessive notifications if I'm not in the room, for apps I'm not interested in.

"Spheres of interest", as you mention them, become less important as you grow the company. We've found it important to not get too sucked into every little aspect of the company- it's too broad. If someone thinks you should know something that's happening in a room you're not in, they can just ping you and you can hop in and stay up-to-date. That's been a really good solution for us.