I did some benchmarking of PHP compiled with gcc 4.0.1. Results and methodology are described at:

http://wp.wikidev.net/GCC_benchmarking

I eventually settled on -O3 with profile-guided optimisation. This gave a 5-10% improvement over the old PHP 4.3.11 build on all the benchmarks except preg_replace. The preg_replace problem is probably because the benchmark was atypical compared to the profiling data, which was gathered using refreshLinks.php.

-- Tim Starling