I remember seeing a benchmark of normal PHP vs Facebook's PHP-to-C++ converter. It would be interesting to see how this one performs...
I've played around with HipHop and Roadsend PHP for PTS. Mostly this has just been a coding exercise to ensure PTS remains very portable and clean. It's also fun trying out the different engines. To some surprise, I don't think I've ever heard of PH7 previously. However, as soon as time allows I'll give PH7 for PTS a try. It looks like PH7 implements much of PHP 5.3 so I think it should work fine; PTS just depends upon PHP 5.2.0 functionality.
Interesting! I think the main reason why results are so similar is that PHP is probably not the bottleneck, at all, and all the computational time is used in OS calls, etc
HHVM can be much faster than PHP... PTS does lots of OS system calls, but also taking lots of time is lots of XML parsing/generation, in particular. There's also zip/tar.gz compression/decompression, in-PHP computational tasks, and other calculations. Graph rendering with pts_Graph is perhaps the most computationally intense inside PTS.