do CPU-specific optimizations make much difference?

[Oleg]

Hi

Gentoo (a source-only Linux distribution) users frequently claim that their 
systems are much faster thanks to the CPU-specifc optimizations they enjoy, 
while e.g. Debian provides only i386 binaries for all Intel/AMD systems.

Are there any benchmarks available comparing applications like KDE, Mozilla, 
GCC itself, etc. when compiled with e.g. -march=pentium4 as opposed to i386?

Thanks
Oleg