I'd agree with you and Shannow that sa'angreal are multipliers too, not granters of fixed amounts of power. However, if RJ's notes state otherwise, I don't think anything in the text exists to prove it wrong.

Or it could mean that a weak Aes Sedai could not use the sa'angreal at all.
I considered this, then dismissed it since the weakened Siuan used it in tGS.
I can come up with two explanations for why weak-Siuan could use it, and still not crumple the walls of Tar Valon:

1) Maybe RJ had it that if a channelers strength is reduced by means like forkroot, shields, stilling, etc., it does not change what sa'angreal they can use or not use. Given that Brandon said there were specific notes about sa'angreal useage by people made weaker, this may well be true.

2) This is more convoluted. It may be that a sa'angreal will allow anyone to draw amount X of the power. However, for each sa'angreal, a user has to be of a certain strength to be able to draw X amount without their connection to the source being destroyed. Thus, a weak woman like Siuan can use these sa'angreal, but if she attempts to draw its full strength, she will be burned out. It could be that the same is the case with the CK.

This could simply be because Rand is drawing through Callandor minus a buffer, so he's overdrawing, whereas in a circle, Narishma can only draw to its set amount.

Can't agree with this.If this were the case, Elayne would have had Nynaeve hand over her angreal to Caire for using the Bowl. The strength multiplication would have been much greater in that case.