Based on my experience, running APC and caching the XenForo sessions will result into guaranteed fragmentation, your graph shows perfectly what I mean. That translates to slower performance overall.
Personally, I use Memcached + Libmemcached for XenForo cache and APC for the regular php files cache.
Here are my stats:
View attachment 31926
Libmemcached library (combined with php-pecl-memcached) is HUGELY superior, compared to Memcache one.