I just migrated a higher traffic vB forum to XenForo and I'm seeing much more aggressive fragmentation which I'm fairly sure is a result of the session caching and generally higher memory usage. I haven't been able to do much long-term monitoring because I've had to reset the PHP-FPM server several times while doing optimizations but fragmentation seems to hop into the high teens relatively quickly. I do see similar (to my lower traffic install) opcode usage at roughly 2000 files cached after traffic levels out over a day or so.
Does setting a proper file hint help? What about user hint? It will be a while before I can get the appropriate data for a user hint setting. The merits of hint settings in APC seem to be vague and disputed.
I'm not really looking for APC specific info that can be found in docs. I've read them all header to footer a dozen and a half times. I'm more interested in anecdotes from people who've run XenForo over long periods of time between clearing APC cache on medium traffic forums with session caching enabled. My average page load time is below .5 seconds with ngx_pagespeed and APC enabled so it's not like I'm having performance issues, even when APC fragmentation is above 10%. I'm just hoping to have some info chambered if it does become a performance issue and I have to disable session caching.