Long Cache times

We host splendidtable.org and a few other sites at a well known Drupal provider that uses Amazon EC2 to host sites. We found A. we were hitting the limits of our server capacity in terms of # of php workers available, and B. that the cost of adding additional server capacity was cost prohibitive for us so I set about to make everything more efficient ie .. long cache times. The idea being that most content does not change once it is created so why waste cpu cycles on recreating it all the time?

So I went about setting cache times for page cache and external cache to a full day. We have Varnish and Memcache running (and even Akamai in front of that but Akamai still needs tuning). Additionally I installed cache actions modules and set the home page and other landing pages (which are either views or panels) to cache actions's permanent cache.

This site really only gets content updates a few times a week so I added a custom module that implements hook_node_insert and update that clears the views and panels caches, runs cache_clear_all for the updated node and landing pages, and sends custom purge requests to Varnish for nodes and landing pages whenever content is updated.

Last I installed Elysia cron and set node and system cron hooks to run just once per week.

My assumption was that a full day of cache times would increase Varnish hit rates dramatically but I have only seen a small increase so far. (Prior to this external page cache or Varnish was set to last one hour).

Here are the numbers this was implemented on 8/19 around mid-day

Date    Domain                 Requests Uncached    Drupal     Varnish Cache Hit Ratio
8/20/13 www.splendidtable.org   149,381 44,343  25,205  83.13
8/19/13 www.splendidtable.org   211,830 58,505  39,331  81.43
8/11/13 www.splendidtable.org   184,868 67,084  40,233  78.24
8/6/13  www.splendidtable.org   152,832 52,348  29,391  80.77

So I don't have much of a sample size yet but it looks like this strategy is only slightly better set Varnish to last for an hour.

The traffic for this site is about 25% hitting the home page or landing pages and the rest kind of a long tail spread across stories and recipes on the site.

What do you think? Have I chosen a caching strategy that really isn't much better than just cache for an hour? Should I try for an even longer cache times? is there something else I am failing to take into consideration or misunderstanding? Maybe I should concentrate on caching at the Akamia level? I also think this may understate the effectiveness of memcache as some request may go to the Drupal level but skip hitting Mysql.

Really my first take was 80% cache hit rate is pretty good so just add more RAM so we can have more php processes but thats cost prohibitive under our current hosting arrangement.