The gentoo folks would definitely have you believe otherwise. A friend of mine swears that rebuilding everything using the -march=athlon-xp results in a much more responsive X subsystem.Athlon reorders instructions internally anyway, the net result generally being that it makes no difference what CPU instruction scheduling is chosen, as the chip does it's own thing internally.
I'd be kind of surprised if compiling with all those options yields any real world performance gains anyway. Placebo effect perhaps... ;o)