I agree that the bottom rung of AS are vastly weaker than the top tiers. I just think the main characters in the series are much closer to the top than to the bottom.

Nynaeve is in the top 1% of all women in strength, I'd even postulate that the female forsaken could all be in the top 10% with very little difference in total strength... I won't claim I think that's a fact, there's enough evidence to show that there could be as much as a 15% spread if you include Moghedien.

If there was all that much difference between Lanfear, Graendal and Mesaana it would have been noted by the BA women who have felt their strength. Sheriam only notes that there is a "very strong" woman outside of her tent, but she doesn't know which woman it is so IMO their strengths are close enough to be difficult to judge between them. If Mesaana was 20% lower than Graendal Sheriam would realize which FS was standing outside. Likewise, Alviarin notes that Graendal and Lanfear both allowed her to feel their strength, but rather than postulate that Mesaana must be weaker, she immediately jumps to Mesaana is posing as an AS.

Elayne's potential is probably about 25% weaker than this level (90), which puts her around a 65... Cadsuane another 15% below that would make her about a 55, Moiraine 15% below that makes her about a 45... then the smaller levels between AS start, Sheriam could be a 40 (10% less than Moiraine)... but by the time you get down to the bottom level AS you are at about a 15 (Daigian could be a 10), but there.