For our sites running Bolt - a PHP CMS which makes heavy use of autoloaded classes - we saw a performance improvement between 45 and 55 percent. For Wordpress, it was around 30-40%.
Though I guess that is really a composer issue, the inability to not use something like curl when allow_url_fopen wasn't available seems insane to me. allow_url_fopen is not a great thing to have enabled in production.
That said, a faster parser and intepreter would do autoloading faster as well, so it may not be opcache at all and just by virtue that the autoloaded classes are interpreted faster.