Good question, so far since xCache has been disabled I haven't had any problems.
I am however thinking about a new strategy that doesn't envolve xCache. I want to try eaccelerator with PHP5.3 and keep PHP5.3 while it still gets security updates and then update directly to PHP 5.5 with Zend opcache.
I'm under the impression that xcache is good for a few sites but not for hundreds of sites and eaccelerator seems to have been tried and tested on large multi site servers.
I'll still have to test but I'm thinking about allowing 10 to 15GB (and maybe more if it takes it) of shm and disabeling disk cache. If I can't get a high SHM to work I will create a 20GB tempfs partition and set eaccelerator to use this.