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

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
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.