What does 15x really mean? In-Memory sounds like MySQL's innodb_buffer_pool_size being set big enough with innodb_flush_log_at_trx_commit set to 0. MySQL 5.7 with some expensive off the shelf hardware can do over 1.5 million queries per second to the same table; this number is also kinda out there but at least it tells me some real numbers and explains how to replicate it http://dimitrik.free.fr/blog/archives/2015/10/mysql-performance-yes-we-c... (noted that this result is with the query cache disabled).
Due to all the performance improvements to MySQL we no longer need a key/value memcache like store, we just use the database; drupal.org also did the same https://twitter.com/mcarper/status/720063584185626624.
This post makes some bold claims; please back them up with some real world comparisons :)