[Lainadann]

Whoever wore the one had power over all the other rings but relative to their own power they had to begin with. The realms of Lorien and Rivendell were preserved by the power of the rings so it stands to reason that Saruman being one of the wise, should he have gained the ring for himself and being in origin more or less equal with Sauron (interstingly both of Aules household) would have become as powerful as he and be able to defeat all before him. Saurons power was mostly in the ring itself and he feared any who had the strength to wield it against him.

I'm just speculating of course but it's possible that the ring would in a sense allow the full expression of Sarumans power that, being incarnate in flesh, he could never otherwise achieve.