Here is a complete /etc/make.conf with optimizations.
http://toya.net.pl/~vermaden/text/make.conf
Generally KERNEL suffers when you force it to use MMX or other complex instructions, so they are disabled, because of performance for the kernel itself for sure, even if you specify them in /etc/make.conf the build system will disable them because its no good.
On the other side userland apps like Mplayer take quite big advantage of these, so you should use them for userland apps, but I did not checked if Mplayer is build with them by default for example.