We currently run roughly 300k pageviews/week with a 50k user base without any performance issues. Contact me for direct advice, and I will try to reserve time to write additions to the performance documents. 15-20k users on quizzes is definitely possible with optimization and if necessary loadbalancing among multiple servers.
the slowest part in Moodle is the database abstraction, which in my opinion, is faily poorly designed. For instance manipulation of database data often takes two roundtrips: fetching data from mysql an array, transforming the array in PHP, preparing the statement for insert/update into the database and then writing information to the database, and then retrieving it again to show on your screen (with array processing)