I do performance WP hosting and would like to try out EFS but it is not available locally yet. What I do is use redundant RDS, ELB etc but on all the WP installs have an inhouse plugin that with any change it ioniced rsyncs that vhosts tree to all webservers and ssh's a command to them all to dump their WP cache. It may seem hackish but the sites absolutely fly, have tried several clustered file systems but all that…
And if your cache is all on disk, a very quick win would involve moving that to memcache, no? That also makes forced expiration very easy.
Of course there are managed services for all of these things anyway.