I guess it's more straight forward to setup memcache within config.php, but maybe there are other core xenforo tables that would benefit from memcache?
This is from the documentation https://dev.mysql.com/doc/refman/5.6/en/innodb-memcached.html
14.18 InnoDB Integration with memcached
14.18.1 Benefits of the InnoDB / memcached Combination
14.18.2 Architecture of InnoDB and memcached Integration
14.18.3 Getting Started with InnoDB Memcached Plugin
14.18.4 Security Considerations for the InnoDB memcached Plugin
14.18.5 Writing Applications for the InnoDB memcached Interface
14.18.6 Using the InnoDB memcached Plugin with Replication
14.18.7 Internals of the InnoDB memcached Plugin
14.18.8 Troubleshooting the InnoDB memcached Plugin
buffer pool mechanism. Data modified through memcached operations such as ADD,SET, INCR are stored to disk, using InnoDB mechanisms such as change buffering, the doublewrite buffer, andcrash recovery. The combination of memcached simplicity and InnoDB reliability and consistency provides users with the best of both worlds, as explained in Section 14.18.1, “Benefits of the InnoDB / memcached Combination”. For architectural details about how the components fit together, see Section 14.18.2, “Architecture of InnoDB and memcached Integration”.