I'm somewhat inclined to say the early fourteenth century, especially in the enjoyable scenario of a conquest/merger by the Serbians. A state covering much of the Balkans plus fertile western Anatolia and maybe the Chersonese and Cyprus will have just as many resources as OTL Italy, and probably considerably more diplomatic clout throughout the Orthodox world. That would be a "Great Power", no doubt.
As for a superpower, you probably need a pre-Manzikert PoD.