Well, I'm sorry for saying you were degrading Nynaeve's strength, but you also stated that Mesaana was stronger than Nynaeve. And I find it difficult to believe that Graendal, who can call Mesaana a second rate Forsaken, is weaker than her. Equal to Semirhage? Yes, maybe. Weaker than Mesaaana? No way.
So, if Semirhage and Mesaana are stronger than Nynaeve, and Lanfear and Graendal are also stronger, Nynaeve just cannot be average female forsaken strength.

No they don't. No one has ever said Sharina is incredibly strong, they've only said she has remarkable potential.
And they certainly care about what's there and what's not. Otherwise Moiraine and Siuan would have been able to lord it in the Tower from the beginning. Instead, their current strength mattered, not where they'd be finally.
The same is the case here. Knowing how incredibly strong Nynaeve or Sharina would be doesn't in any way measure up to actually feeling that strength.
Nor do we have a PoV from Sheriam when she was measuring Nynaeve's strength. Perhaps she'd have used "such strength" to describe Nyn's strength too. After all, Sheriam is now considering that the Great Lord could have other servants with such strength, meaning she no longer thinks only the Forsaken can have such strength. And where could she have gotten that notion than from being around the wondergirls and Sharina?

I hate that we can only guess. I think I can count in the fingers of one hand the number of quotes relevant to OP strength in this book. While extraordinarily satisfied with it on the whole, this is one aspect that I'm unhappy about.

Doesn't work. Nynaeve sensed channeling from her ter'angreal that she couldn't see or detect. Which is why she wondered how the Seanchan knew about hiding their ability.
Immediately after that, Cadsuane's weave disruptor was directed on Semirhage, which removed her illusion as well as her hidden ability. I doubt Semirhage had the time to weave a fireball and also a reversed web to seemingly reduce her strength (why do that?).

Incidentally, Melaine says that only Cadsuane could have matched what Aviendha did. Does this mean Aviendha is currently at Caddy's strength? Because that would mean she isn't far from her potential strength.