Rogier,
I asked an engineer, and here's his response...
"I don't have a good answer for default memory usage expecations. It probably depends a lot on what the users are doing. Also, performance issues are difficult/tricky to debug and can have many sources besides memory usage, so even if I could answer it, that might not be helpful. For users seeing performance issues, I'd encourage them to open tickets with support and provide as much detail as they can about what the users seeing perfomance issues are experiencing specifically, and what they're doing during the day. Are these agents, supervisors, everybody? Any plugins installed? That rabbit hole goes pretty deep.
Also, just to be clear, I'm not trying to dismiss performance issues. We absolutely want to make improvements to the applications performance, but in a product as big as ours we really need good data to work with so we can isolate specific problem scenarios to tackle."