Last-Modified header is changed, even if the resource hasn't changed
The code that is causing the issues you're describing lives in drupal_serve_page_from_cache(). Instead of using a hash of the contents of the cache; Drupal uses the $cache-&gt;created value. Looking over everything hacking core is the only option at the moment. I'll see if there can be a way of not hacking core if one has total control of the cache backend; if there is a way look for an update inside of https://www.drupal.org/project/apdqc