Hi Fredrick, thanks for the response.
APC hasn't had a stable release since May 2011 and 3.1.4 had to be pulled for a nasty bug. Development on it is slower than a WP site not running W3TC. And APC is even less compatible with PHP 5.5 than it is with 5.4 - I traced the source of my 500 errors to APC with PHP 5.4.
http://svn.php.net/viewvc/pecl/apc/trunk/?view=log
Thanks!
-J