Moiraine will have to succeed in establishing her dominance primarily through the force of her personality. But among Aes Sedai, inferior strength in the Power will make it all the more difficult to establish supremacy.

Being stronger in the Power is a pre requisite. Using that to hammer down her dominance will then be up to her.

Hence, starting off with say double Cadsuane's strength will give Moiraine a huge advantage from the word go.