This sounds very useful, especially if you consider that PHP is used in embedded environments like MonoWall and pfSense.
My big question for larger installation is this: xcache already does a great job at bytecode caching. The largest slowdown in the majority of PHP applications is the relational engine sitting underneath it. After much profiling of previous applications, I have always found the biggest benefits to application performance in taking a casual schema to a rigorous level. And if that is not fast enough, then you throw in some memcached to the mix.