Node.js + socket.io determine maximum amount of clients per instance

I am asked to perform rough calculation of how many connections can single socket.io instance handle on a large EC2 instance (16Gb Ram, 4 Core Xeon).

I updated sample of simple socket.io benchmark.

Serever:

var io = require('socket.io').listen(80);
var exec = require('child_process').exec; 

io.configure(function() {
  io.set('log level', 1);

  var transport = process.argv.length >= 2 ? process.argv[2] : null;
  if (transport) {
    io.set('transports', [transport]);
  }
});

var payload = "Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item " +
"Payload: Payload line item ";

var users = 0;

function roundNumber(num, precision) {
  return parseFloat(Math.round(num * Math.pow(10, precision)) / Math.pow(10, precision));
}

setInterval(function() { 
  io.sockets.emit("message", payload);
  console.log("Users: " + users);
}, 3000);

io.sockets.on('connection', function(socket) {

  users++;  

  socket.on('disconnect', function() {
    users--;
  })
});

Client

var io = require('socket.io-client');

var message = "o bispo de constantinopla nao quer se desconstantinopolizar";

var receivedMessages = 0;

function user(shouldBroadcast, host, port) {
  var socket = io.connect('http://' + host + ':' + port, {
    'force new connection': true
  });  

  socket.on("message", function(){    
    receivedMessages += 1;
  })
};

var argvIndex = 2;

var users = parseInt(process.argv[argvIndex++]);
var rampUpTime = parseInt(process.argv[argvIndex++]) * 1000; // in seconds
var newUserTimeout = rampUpTime / users;
var shouldBroadcast = process.argv[argvIndex++] === 'broadcast' ? true : false;
var host = process.argv[argvIndex++] ? process.argv[argvIndex - 1]  : 'localhost';
var port = process.argv[argvIndex++] ? process.argv[argvIndex - 1]  : '3000';

for(var i=0; i<users; i++) {
  setTimeout(function() { user(shouldBroadcast, host, port); }, i * newUserTimeout);
};

setInterval(function() {

 console.log("Messages received: " + receivedMessages);
 receivedMessages = 0;

}, 3000);

I ran this sample, client was started on medium instance, server on large. And I got result amount of received messages around 4500 per 3 seconds.

But the network, cpu and ram usage was in range of 2-10%.

Is it possible to increase the number of served connections?

Server OS is Windows Server 2008 R2.