Now since PHP 7 alpha is released on 12 June 2015, so definitely Moodle has to be tested with it to review the performance and other issues.
Usman Asar, one of the most Particularly Helpful Moodler (PHM), has tested Moodle with PHP 7.0.0 alpha and mentioned a considerable improvement of up to 30% or more in memory usage and rendering time improvement with PHP 5.6.7 (zend opcache enabled in both cases). Along with the improved performance he has also experienced many errors while testing.
Now since Alpha 2 version is scheduled to be released on 25 June 2015 then we may get some stable performance stats including running the Behat and PHPunit tests.
You can also join in testing Moodle with PHP 7.0 from this Moodle forum thread: https://moodle.org/mod/forum/discuss.php?d=315158