Website Optimization Ways, Need Criticism

Given: not a very large portal, 7 servers serving it in two data centers, a new approach to its telematic support has already been almost completely tested, I would like to hear criticism.

so ...

General: the
site is written in php5, uses mysql as the DBMS, apache2 as the backend, nginx as the frontend, memcache as the cache, in some cases smarty caching is used (a full static copy is created for unregistered users and search engines), eaccelerator is used to php5, the template engine is smarty


DB:
master-slave replication is used, both data centers use one master, there are slaves in each DC, if an extension is necessary, an additional slave is installed

Files:
One file storage is used to which all static files are written, the return comes from cache servers (nginx through proxy_store (interception 404 and subsequent download) from the storage, files on cache servers are killed by atime, the portal core generates unique names for all static files, so that proxy_store works correctly) there can be many cache servers, load balancing is based on views & round robin bind, the advantage of the approach is that you can install cheap servers, you do not need raid for cache servers, you can achieve speed by spreading files several times drives.

Core:
The kernel can handle errors of loss of the master (limited functionality) and slave (everything is bad), can use cache requests in memcache for registered users and smarty'evy caching of the whole page for search engines.

Which direction should I dig for optimization?