I don't think it's logical. Stock PHP can easily handle a website with 250k monthly visits. Or, rather, your speedups will most likely come from talking to the database less, doing less IO, that sort of thing. CPU probably isn't your bottleneck now.
At my last company, we handled about 20 million API requests a day through a PHP API. Granted, the request and response were smaller than a standard webpage, but that was using stock PHP (hell it wasn't even custom compiled, it came straight from the Debian packages).
So, you need to hit a pretty huge amount of traffic before something like this makes sense. Of course, it's always neat technology to play with.