If you hadn't seen it already, the TokuMX team is putting together a storage engine module designed to work with vanilla MongoDB's new pluggable storage system. This blog post[1] compares TokuMX to vanilla Mongo running their (experimental) engine. The short version is: TokuMX made some changes outside the scope of the pluggable storage system that give it the edge in some circumstances. An interesting read.

[1]http://www.tokutek.com/2015/01/first-tokumxse-performance-nu...