I nearly made a post about this, but decided not to because Brandon himself said he was unsure how they worked. According to Brandon angreal may be multipliers but sa'angreal provide a fixed amount of the One Power.

I also don't agree with this. The first evidence we have of this is Siuan saying that a strong Aes Sedai could crumple the walls of Tar Valon with Vora's sa'angreal. That means that a weak one would not, and therefore not all channelers gain the same amount of saidar from the sa'angreal, which strongly indicates that Vora's wand is a multiplier.

Secondly, when Elza draws on Callandor through Narishma, it glows like a flame. When Rand uses it, it is so bright that you can't even see that it's a sword. It's another indication that Rand gains more from Callandor.

As for Egwene, it's possible that if the leader of the circle gets hold of a sa'angreal or angreal, the whole circle is augmented. This is further substantiated by the scene where Siuan Heals Mat while using Vora's wand, and in the end every Aes Sedai is tired and sweating - another indication that they all drew on great amounts of saidar for a prolonged time. I'm not sticking up for this theory too much, but I do think it's a possibility.