Chris Merrill has written up a case study measuring the load handling abilities of the Zend Optimizer with a default application - in this case, SugarCRM.

This article measures the performance impact of the Zend Optimizer on a real-world processor-bound PHP application (SugarCRM) under load. Our measure of performance is user capacity. We define that as the number of simultaneous users that the system can support while meeting the specified performance criteria. The performance criteria for this test require that all pages load within 6 seconds and no errors are encountered in the application.

The process is broken out into a few different sections including the methodology he used (including the software list, hardware used and configurations of both) and the results/analysis from running the test requests (such as page duration and server process use).