I really don't see Node outdoing PHP at any time. Both are very different beasts. Node has its advantages and PHP has theirs. For instance, take a look at CouchApps. Man, those things are awesome, simple, infinitely scaleable and super easy to deploy. One could suggest that CouchApps will outdo Node. However, I doubt that, too :-) In reality, All three are viable options for many projects, but its just as viable to use all three in one app.
I do a shed load of Node.JS development, and it's fun stuff. Node is great for Cloud dev, handling huge numbers of connections and interacting with databases. However, I wouldn't like to use it for heavy file access. In fact, file access can be a big bottleneck for Node. PHP on the other hand is great for file access. Building CMS's in PHP is far simpler and more useful than doing so in Node.JS... Just direct me to a single CMS platform for Node! I'd struggle to find any that were any good. This isn't really because Node hasn't been around that long (even two years is long enough to build a simple CMS), but because a CMS in Node would be an altogether different beast.
My opinion? Use Node alongside PHP. Use Silverstripe to handle the CMS, while Node handles the connections. This is perfectly feasible and very flexible.