I guess the question I've been having is that if I want to begin a new project with performance as a main priority, should I use HHVM+PHP or HHVM+Hack over something like Java or Go, or is it more for making existing PHP codebases more performant.
However, if I'm able to hang back and take some time to develop something, priorities like being able to handle X I/O writes or, as you point out, threading becomes more relevant.
I think that it's easy enough to get a site running entirely on PHP--well tested, well supported, architecturally sound, and rewrite services into <noninterpreted-language-of-choice> as you hit performance bottlenecks.
If performance is absolutely critical to your app, then you may consider writing everything in Java/Go/C++, but it's pretty brutal for doing templating and whatnot. Besides, once you generate the page, there's a good chance you're going to cache a good part of it anyhow!
So if you have the chance to start fresh: choose Java or Go. Or anything else... (define "performance"?)
Node/IO.js and Go have their advantages, especially if one wants to use WebSocket.
Nowadays, half of the code is client side anyway (be it HTML5 JS (maybe with React/Angular) or mobile apps) - in such cases the server output and receives a lot of JSON. And then Node.js/Go/HHVM are about equal as good in benchmarks, at least in my personal benchmarks (10.000 -100.000 requests/s with Apache AB tool).