Basically there is no more efficient way to do it than via XMPP, FCM is also an XMPP service. So the best is really to use Conversations as an UP end-point to get notifications from other apps. AFAIK it doesn’t use a websocket, but a native xmpp connection for it, which is even more efficient.
The reason Conversations also supports FCM is that some Android phone vendors are extremely aggressive in putting apps into sleep mode and only FCM notifications work reliably on those.