James - Even though the cache performance test page shows that MongoDB is only marginally slower than Memcache for reads, profiling indicates MongoDB reads (using the same cache server) are about an order of magnitude slower than Memcache. I tried playing with MongoDB configuration, but nothing seemed to help. So, I am adjusting course. I am now in the tedious analysis of determining which application caches can co-exist in the same Memcached instance without having performance problems due to purging.
I am not very far into this analysis, but I have found that the course contacts cache purges every hour (CACHE_COURSE_CONTACTS_TTL), so that's an obvious candidate for isolating from the other caches.
Also, the language string cache and the available languages cache both purge whenever one saves a change in the language string customizations.