Sabayon developers starting rolling out systemd today aiming to replace udev, but not displace openrc that will remain the default init system. The plan is to support both init systems for a reasonable period of time, allowing systemd to be implemented progressively and with assiduous testing to avoid risking the functionality of the system.
The reasons behind this are mainly performance improvements as you’d expect. According to benchmarks that the leader of the project Fabio Erculiani performed, systemd is faster than openrc, and with systemd being also more feature rich, the decision to start supporting it was easy.
To test systemd on your Sabayon 11 installation you need to use eselect-sysvinit and eselect-settingsd and switch between systemd and openrc at runtime. The less brave should wait for systemd to mature in Sabayon and get it in the near future through the usual rolling release updating.