So after a couple of people pointed out my naivety in configuring Rails, I decided to re-run the tests. What I did was configure Rails with a 10 mongrel cluster and the Pound load balancer as per Jared's recommendation. However, to make things more equal I reduced the Grails Tomcat server's thread pool down to 10 by setting maxThreads=10 in Tomcat's server.xml.
The result was that Rails' performance degraded in all except the long running query test, whilst Grails' performance significantly improved in all except the same test. Clearly, since I have only dual core's on my MacBook giving Rails or Grails more processes doesn't necessarily improve things for the shorter tasks. Check out the updated benchmarks.
Again, I'm no Rails performance tuning wizard so if any Rails expert can suggest improvements to the Rails configuration please don't hesitate to shout.