I would like to find out from anyone that has built a high traffic Drupal community website ( a website with at least 500,000 visitors a month, with anywhere from 10,000 to 20,000 authenticated users at any one time ), is it is possible to create this kind of site without having to do a high level of hacking to make Drupal scale? Can standard Drupal scale without a lot of query and other types of hacking / coding?

Can I install Drupal, configure the modules I need, and then add more hardware and functionality (APC, Varnish, Memcache, etc) without having to do query hacking? How do people deal with Drupal's high number of database queries without hacking, or is there no way around having to hack? If you have built this type of community site with numerous authenticated user, what were the biggest hurdles and stumbling blocks when it comes to performance and scalability? I already know about auth cache and pressflow.