I'm not aware of any official benchmark, however, JDBC is a direct link to a database and as such, chances are that it provides better performance. On the other hand persistence mediated by JPA/Hibernate can be fine tuned by using caches, for example. There is not always a clear winner as it mainly depends on the structure of your database, on the kind of queries used. Have a look here: http://docs.jboss.org/hibernate/core/3.3/reference/en/html/performance.html

Should you be interested, in JBoss Performance Tuning there will be a whole section dedicated to improving the performance of data storage.

hope it helps,

Francesco