Is it redundant to use the "boost" module if varnish is used? We run our Drupal instances behind a Varnish server, but we're looking at alternative/additional ways to speed up the site. Boost is a module that is frequently named for this purpose; it appears to generate static versions of pages and serve those to avoid regeneration time, but—if we're already caching with varnish—does this give us any benefit?
No, with the correct configuration, Boost and Varnish can work together. The combination is said to speed up your site even more than just Boost or Varnish (I haven't tried it myself yet).

See Will Boost conflict with Varnish and Use Varnish for cache storage instead of file system. If you're visiting DrupalCon London, check out the Boosted Varnish talk.