Works pretty well for me on my MacBook Pro, but on my G5 it performed
abysmally.  In fact, it ran so much worse that I cleaned up my sandbox 
and did both checks all over again to make sure I didn't mess something
up.  It looks like my MacBook Pro saw about a 14% degradation.  Both
computers are running Mac OS X 10.5.6 with the latest Xcode - 3.1.2.
On both computers gcc -v reports 4.0.1, Apple build 5490.

If this is applied to the core I think it will have to select for more
than just gcc.  It will also have to select based on the chip
architecture.