Spilling into another dimension isn't quantifiable.
And a name doesn't mean anything. Unless Vegeta's Big Bang Attack can destroy the universe....
How is it not? If an explosion is strong enough to shatter space itself, then holy shit, its powerful. That's Ultimecia level power. Whats hard to understand about that?
My eyes. Or are you saying Sephiroth wasn't holding them all at the same time in his bae adn thus dividing up his mental power?
He was, but how does his telekinetic will get divided up? Where does it say that he had to divide up his attention and that weakened his hold? Where are you getting that they worked together and were somehow able to combine their will to overcome his? You're reading this stuff on your own. We're shown Sephiroth holding each of the characters with his will, and then they all break free of it one by one. Period.
He was also holding back Holy which I'm sure took even more focus away from the heroes.
What focus? It's his will. His desire. His will and desire alone is enough to subvert and hold Holy. Where the hell does it say he was distracted or needed focus? It's his will, not concentration.
Mako, think of it this way and you'll see how the wing business is merely an aesthetic choice. Safer Sephiroth is weaker than ACC Sephiroth, but in AC Sephiroth has not just one missing wing, but VII. He has all of his Safer power and then some, but he still chooses to manifest without a single wing.
His wing is still indicative and apart of his strength and transformation. It is not just aesthetic. The CC Complete Guide references Hollander's wing's size as an indicator of his strength. It states why his wing is tiny, compared to Genesis's which is huge. It's a reference of power. So it being just aesthetic is out of step.
As for why AC Sephiroth is stronger while having less wings..it's never stated that more wings means more power. Sephiroth donning his wing is a reference to his transformation and power up. That's all. I or the game, never said that the more wings you have, the stronger you get. It's just a physical manifestation of Sephiroth's true power.
Also, Genesis' degradation making him more monstrous and powerful has nothing to do with his wing. His degradation is cured after Banora and he hides his wing again, but DoC shows us that even without degradation the wing is still there.
The wing is a symptom and result of said degradation and transformation.
And we really don't know how the lifestream was able to cure his degredation. We only know it was cured. It never is stated that he lost all of his powers as a product of Jenova Project G.