Like Fionwe, I think there are many meanings to each quote. When Cadsuane refers to Semirhage being one of the most powerful women who ever lived, I don't see really how strength could enter the debate. Strength in the Power does not make you invulnerable to torture and being broken. To me it's more a reference to Semirhage being one of the top leaders in the Shadow - a woman who's done so much evil that there could be nothing that could intimidate her (clearly Cadsuane hasn't met Moghedien).

I think it's reaching to assume that these quotes mean anything, and I did keep a look out for them when I read the book. Semirhage is at the very least as strong as Nynaeve, and that still makes her one of the most powerful channelers in the world. If one adds her major Talent, her lust for pain and her evil intent, it's easy to see why there are very few people who can overcome her. Making her as strong as Alivia will not really add to this.