I might be wrong here, but if you're using long polling in your webapp, shouldn't you be using a randomized wildcard subdomain anyways? For example, Quora uses #.tch.quora.com as their long polling hostname, with the #-part randomized for each of the browser tab, which effectively allows for an unlimited number of concurrent connections to a single IP address.

I mean, even though Chrome allows 6+ concurrent connections to a single hostname, nothing's stopping users from opening 10 tabs of your application.