Yes, control over the Lifestream is the only way to be really powerful.
Everything in FFVII is connected with the Lifestream, and that includes physical strength and speed. This is why Weiss was so fast(in his fight against Chaos Vincent) and so strong(blocking bullets with his arms) after merging with it. And it's one of the reasons why SOLDIERs are so strong and fast.
And IIRC, some of the Limit Breaks are caused by a burst of adrenaline, where the person can use his/her spirit energy(the "Lifestream" that everyone have inside themselves, like Vincent explained in AC) to boost his/her abilities.
Everything in FFVII is connected to spirit energy and the Lifestream. It's the source of life, for god's sake! It's impossible to be as strong and/or fast as someone who have controll over all of it.
And it's pretty obvious Sephiroth have controll over "all" the Lifestream(or most of it, because Aerith was still there). There is nothing above him, and that includes Minerva, the living will of the planet. Or you will say now, that the consciousness of Gaia don't have control over the Lifestream?
This is why he can control "all" of it. And IMO, power of the Lifestream, and power of Jenova, give the victory to him.
Note: Weiss DIDN'T have control over all the Lifestream, because Omega was formed of only PURE spirit energy. Stagnant spirit energy is part of the Lifestream too.
And don't tell me she/it has stagnant energy, because of Nero's fusion with Weiss. Nero's stagnance is probably a fraction of all the stagnant spirit energy in the world.