Don’t know if it helps, but the developers have said that all other aircraft (either traffic or multiplayer) is limited to a 200km radius. I am fairly sure that you see the nameplates of all aircraft in that radius, so unless there are more than 50 planes around you then the effective limit is 200km.
I agree that listing the distance from the user aircraft beneath the nameplates would be useful. Flight Simulator X would cycle all the information to prevent clutter, and allowed you to choose what information to display.