It sounds like it, at least I can't really see how life expectancy could progress linearly (and thus continuously within a level as well) along the "levels" unless the levels are merely markers for equal arbitrary divisions on a linear and continuous scale of strength.

Thus the difference between Lanfear and the first larger group (the one with Elaida, Romanda and co.) is similar to that between the present ruling class of the Tower and the largest group of middling sisters, which itself has a similar gap with the weakest Aes Sedai. And those "weaker Aes Sedai" already start being rarer, if not as rare as women like Morgase and Sorilea, let alone women weaker than that who we never saw.

How impressed the sisters are with Forsaken is easily explained by the Tower hierarchy, an inheritance from days when much stronger women were in the Tower. Nowadays it's almost an absurd system, with the gap between Elaida and Daigian being not that great. But still... women like Elayne-Egwene stand, by the hierarchy, well above Elaida, so they impress a lot.. but they're not trained, not really sisters, so for now this is very undermined in comparison to the effect Cadsuane can have. All sisters middling and above have developed some self-confidence by the fact it's ingrained in their bones they're superior to women they see as weak. When they see women like Mesaana that probably stands as much above Elayne and co. and Elayne stands above Elaida, they're overawed, their self-confidence goes out of the window. Such a woman in the Tower would make all the other crawls to her orders, not a single sister would be even close to having a right to be listened to by her. And then, Mesaana carries the aura of all the knowledge of the AOL with her. I guess it's important to remember that it's women who exaggerate the value and advantage of every little gap in strength all their lives that judge Forsaken to the same standards.

The Wise Ones, who don't especially value strength in the OP or see this as the mark of someone superior in every way are much, much less impressed by the strength of Forsaken, and by ji'e'toh standards they're just worse than dat'sang. They already have little better than scorn for AS ethics, so for Forsaken...

I suppose it's higher than 5% (but I've been told nothing about this).

In fact, I suspect the actual formula for circles is more complex than a constant strength1 minus x% + strength2 minus x% valid for all pairs. I think balance comes into play, and possibly in a two women circle being fairly close in strength leads to losing less strength than if the two women are very far apart. Of course, we can now suspect that even Daigian is only relatively weak.