I don't have my books available right now, but I believe that in The Dragon Reborn (or was it The Great Hunt?) Siuan told Egwene and Nynaeve that with Vora's sa'angreal (though she didn't mention it by name) the weakest novice could make the walls of Tar Valon tremble. So, had Egwene given the sa'angreal to Nicola, perhaps the circle would have been much stronger. But power sufficient to cause the walls of Tar Valon to tremple was probably quite sufficient. And perhaps Siuan's comment was what made Egwene use the sa'angreal herself.

Or it might be as mentioned in this post:
[quote]Brandon said at the SLC Q&A, in response to a question about whether the Choedan Kal magnifies the user's power, that "my understanding is that it is a reservoir of power. It is not necessarily a magnification. A very weak person with a very powerful sa’angreal is very powerful." So rather than magnifying the channeler's power, a sa'angreal seems to allow the channeler to hold a total amount of power. That is, a sa'angreal grants the same amount of saidar/saidin to the user regardless of the user's strength. In contrast, an angreal seems to magnify the power available to the user. [/quote]