Having max/min spare servers as the same amount is a bad idea. This is incurs a substantial amount of process swapping, as every single request php-fpm is going to try and ensure there are precisely 256 idle servers.

I could be missing something, but it looks like you are using the default settings. Have you tried tweaking it all? Specifically setting apc.stat=0, which will stop it from checking the mtime. You'll need to clear the cache with apc_clear_cache() when you make code changes though. You may also want to look at apc.php to check for fragmentation and adjust apc.shm_size if necessary