The solution was easier than I'd have thought. I complete upgraded Sabayon, via Rigo Application Browser. After that, I did not have any trouble with these commands:
# modprobe vboxdrv
# modprobe vboxnetadp
# modprobe vboxnetflt
and could successfuly install VBox.
So, to summarize, before following sabayon's wiki on VirtualBox setup, do upgrade. At least it worked for me.