Nitpick: Your numbers are a little off. Rails/REE tops out at around ~300 reqs/sec on a current commodity box (16 cores/16G). This is of course application-dependent and optimization can squeeze it some, but it's a different ballpark.
When you look outside of ruby-land you can indeed find frameworks that will handle your 6000/sec on the same hardware (e.g. twisted, node, some of the evented java-frameworks).
So, depending on what you compare to rails, the difference can easily be an order of magnitude.