Node and its ecology of libraries are asynchronous from the ground up. With Twisted or EventMachine, you'll eventually call a library that performs blocking I/O (probably sooner than later) and that will be your bottleneck.
With Twisted at least there are usually asynchronous versions of commonly needed libraries. Given that, it sounds like they're probably fairly analogous then, right? I assume Node will have the same problem the moment someone grabs a package that's meant for a Javascript framework that isn't explicitly asynchronous.
