While the rails benchmark tries to compare different template engines in the same language, framework, and with blocking IO, you're comparing these results to rendering under "pure" Node.js (no framework that slows you down) with non-blocking IO.
Don't take this the wrong way, tho, I'm absolutely blown away by your numbers!
But I'm really interested in eliminating as much of the framework as possible to get more comparable numbers even to other platforms.
I'm also sort of blown away by the throughput node allows you.
https://gist.github.com/3905579
Take note that the margin of variance on these numbers is several hundred, so don't read too much into small shifts ... but:
With Express, and "c=1", I get 1,876 requests per second.
With Express, and "c=10", I get 2,730 requests per second.
There isn't meaningful overhead imposed by Express for this particular simple template rendering.
Worth noting the tests are being run against a local copy of - http://docpad-kitchensink.herokuapp.com/ - which does a lot more stuff than the basic haml rendering.