I have run some benchmarks on develop, straight from GitHub's zip and running on a clean db. Used ab with no concurrency, didn't restart the server between runs, primed the caches with a test run before saving the results. PHP 5.5.6, /tmp is tmpfs.
I have tested three configurations:
2. with -o
3. with -o plus using Composer's PSR-0 autoloader for the phpbb namespace and without creating $phpbb_class_loader
With Opcache, the results in requests per second go like this: 68.35, 71.37, 77.28.
Without Opcache: 13.73, 13.42, 13.40.
With an opcode cache, using a static classmap measurably improves the performance. Removing phpBB's own class loader improves it further.
Without an opcode cache, the time spent parsing the classmap clearly negates its benefits. Making the classmap bigger negates the benefits of removing phpBB's own class loader.
I'm attaching ab's output for each configuration as t1/t2/t3. "p" is for runs without Opcache.