Doru Moisa has written up a new post with some benchmarks comparing static calls versus singleton calls for a few different situations.

n the past several months I've been working with a rather large application built with symfony. I noticed that symfony makes heavy use of the Singleton pattern (other frameworks, like Zend do that too); everywhere in the code [...] Notice the amount of code needed by the Singleton pattern. Except the [shown] method, all the code in the class makes sure you have only one instance at any time during the execution.

He shows how to replace the standard singleton logic with something more specific and decides to test the two methods, seeing which of them can handle the most requests per second. His sample code is included for both the scripts called and the test script run. In all instances, the static call won out over the singleton instance easily. Even when tested with the Facebook compiler, the results were still the same.