Remember Sephiroth has absorbed knowledge and power from over two millenia -- what magic basically is is the knowledge of the Cetra. Now imagine their knowledge and that of those who came after them all gathered in one being. Makes a MUCH better case for power than anything that's ever said of Jenova IMO...
Well said, but you can't assume that gives him psychic powers. He only mentions that he has obtained knowledge in the game.--- Temple of the Ancients
He says he is far greater than the Cetra, and the only evidence he gives of that is k"knowledge". Materia is condensed life energy. Thats why you can use magic of the Cetra. But Sephiroth was in the Life Stream, which consists of natural, uncondensed Life energy. There is no evidence that one can use the powers of the planet just by being in the life strem for a long time. That is only an assumption. It is an even more unlikely assumption even to the "assumption" that Jenova is greater than you say it is.

How can he absorb the energy of the Life Stream so easily? A strong will? I think he most likely had some help from a greater being--- Jenova.