Mongo in Java can be used with several tools:
- the 10gen official driver
- an alternative async Java driver
- a mapper — a library built on top of a driver — Morphia, Jongo... (see complete list)
Is there some benchmarks comparing mappers to drivers performances and one versus each others?
Unfortunately, there are no benchmarks that I am aware of. The driver home page is here (but I'm guessing that you probably know that) and the Java Language Center containing all information related to Java for MongoDB.
There is quite a bit of work going on with the Java drivers at present afaik but no benchmarks.