Mod-pagespeed is not supported by all PHP apps such as ownCloud. I do normally use PHP OPCache with APCu (for object caching), which i've found out gives best performance for couple of PHP apps i use.
But i've read that mod_diskcache in Apache is better than using the Wordpress caching plugins. It's due to the way Apache's disk cache is implemented (using sendfile API).
I'll hope to get some time to do some testings and post it here.