If you want to compile all packages, this is more easy when you use a regular gentoo or funtoo installation.
for making this in Sabayon, is easier to use the "small" Sabayons, as the CoreCDX only with fluxbox, ex.ftp://sabayon.c3sl.ufpr.br/sabayon/iso/Sabayon_Linux_CoreCDX_10_amd64.iso
as starting point. You should edit your make.conf, plain to use adequate USE Flags etc...
However, for the same graphical env
. the performances os Sabayon and Gentoo are proximal, comparable (I remember that Joost has also carried out tests on this).
Example, my Gentoo installation with XFCE is almost efficient as my Sabayon with XFCE, differences are minimal... Any Sabayon with XFCE is faster than any Gentoo with KDE.
I have in mind that KDE is not a good choice for gentoo. It needs hours to be compiled, and you lose the main advantage of the Gentoo, the performance (Gentoo request many time to be installed and configurated, for lose their main advantage,the speed).