Is it possible to build a high-traffic Drupal community website without hacking?
Drupal can serve high-load sites without hacking. 

You need to use memcache to store Drupal caches not in db but in memory.
For Drupal 6 you need to use Pressflow core.
You need to use Drupal cache mechanism, its configuration much depends on what users do you have most - anonymous or authenticated.

For anonymous users Varnish can dramatically increase site performance.

Don't forget to configure cache for each View and Panel on you site.

Also Drupal 7 can work with MySQL's master-slave architecture.

Don't forget to use cdn for loading static content such as images, css, js and files uploaded by users.

If you will have search on your site - I advise you to use Apache Solr.

I have created several high-loaded sites and haven't hacked any query that was provided by Drupal core. In some cases only queries that were provided by views module were altered - but this is not a hack.

But don't think you can built a high-load Drupal site without deep knowledge of Drupal architecture, API and lot of useful modules.