It would be interesting to look at the performance of the Ruby driver on JRuby 7, which relies on JDK 7's invokedynamic bytecode to improve performance of dynamic languages running on the JVM.

See http://jruby.org/2012/05/21/jruby-1-7-0-preview1.

Perhaps also compare it to jmongo (https://github.com/guyboertje/jmongo), which is a thin Ruby wrapper on top of the java mongo driver.