I updated my previous benchmark of PHP 5.1. This time around I built PHP with GCC 3.4.5, GCC 4.0.2, and GCC 4.1.0.
The bottom line result seems to be that GCC 4 generates slightly slower code:
GCC 3.4.5: 15.223
GCC 4.0.2: 17.896
GCC 4.1.0: 17.847
When I have more time, I will investigate this further using alternative optimizations, including profiling-guided ones.