This is obviously not super precise, but the CPU pegs (load average 1.0, 100% usage) once we get above ~9k messages/second. I don't know how much overhead there is on socket in terms of bandwidth, but some back of the envelope math makes it seem like we're well below network bandwidth on a 100mbit interface. My instinct is that (per rauchg's comment above) that if those kinds of improvements can be made, then we're not yet butting up against a bandwidth issue yet.
As for comparing to other systems, I think we're certainly at the same order of magnitude, but some rough tests a friend of mine ran made it seem like a java server/client pair could send about 4 times as many packets per second. But that's a somewhat unfair comparison - socket.io offers a lot more than simply raw packet transmission and that additional abstraction is going to come at a performance cost. But I think being on the same order of magnitude as a really low level approach like that is a good place to be at this level of maturity of the technology.
As for comparing to other systems, I think we're certainly at the same order of magnitude, but some rough tests a friend of mine ran made it seem like a java server/client pair could send about 4 times as many packets per second. But that's a somewhat unfair comparison - socket.io offers a lot more than simply raw packet transmission and that additional abstraction is going to come at a performance cost. But I think being on the same order of magnitude as a really low level approach like that is a good place to be at this level of maturity of the technology.