LinuxMag has made a performance comparison of Gentoo at various gcc optimization levels against the previous release of Ubuntu (9.04).

http://www.linux-mag.com/id/7574/1/

The X11 and 3D performance comparison is not meaningful, as they compare different versions of the NVidia proprietary driver. But other benchmarks are quite interesting.

Their conclusion:
Quote Originally Posted by http://www.linux-mag.com/id/7574/3/
Although we are not comparing apples to apples, Gentoo did out-perform Ubuntu in almost every test, and sometimes by a fair margin. It does appear that optimizing for a specific CPU can yield a decent performance increase.
Of course the decision for or against Gentoo is not primarily due to performance, as the commenters point out.