I agree with you that Sanderson's perceptions of angreal/sa'angreal are flawed. I think Sidious' "One Power Dynamics" theory from the old wotmania theory post does a great job explaining these concepts. I have it saved on my computer, and would post the relevant sections here, but don't want to without his consent.

Essentially, though, he argues that the amount of power a channeler can draw can be modeled on a funnel. The area of the circle at the top of the funnel is equivalent to the potential of the channeler. He also argues that angreal/sa'angreal add to the area at the mouth of the funnel exponentially, not linearly. An example provided by Sidious from the books is Elayne thinking that her weak angreal will allow her to draw about twice as much as Nynaeve could unaided. Because of this, the amount of additional power that is provided by an angreal/sa'angreal is unique to the strength of the channeler using it; it is not a reservoir of power, as Sanderson describes it. The mathematical formula Sidious uses to determine this is as follows:

radius of funnel: r
potential: Pi(r.r)
angreal: Pi(r+1)(r+1)

Two examples of this applied to characters, based on their comparative One Power strength as predicted by Sidious:

Egwene
radius of funnel: 5
her potential: 78.5
angreal: 113

Nynaeve
radius of funnel: 7
her potential: 153.9
angreal: 201

You can see from this that Nynaeve is almost twice as strong as Egwene when both are using an angreal of the same strength, because angreal appear to add exponentially to a channeler's strength.

Sidious goes on to give some examples of this with sa'angreal. His formula for determining strength with sa'angreal, with approximate values, is as follows:

Channeler
potential: Pi(r.r)
sa’angreal: Pi(r+2)(r+2)
Callandor: Pi(r+50)(r+50)
Choedan Kal: Pi(r+2000)(r+2000)

This yields the following estimates of strength with sa'angreal for two characters:

Rand (radius – 13 – the strongest of all)
Potential: 530.8
Sa’angreal: 706.725
Callandor: 12466
Choedan Kal: 12.7 million

Verin (radius – 1 – a moderate Aes Sedai)
Potential: 3.1
Sa’angreal: 28.27
Callandor: She can’t use Callandor
Choedan Kal: 12.6 million

This makes sense with the descriptions given in the books.

Sidious also tries to explain why the Choedan Kal have a minimum strength requirement based on his funnel theory. His argument is that stronger channelers have more reinforced conduits for channeling larger amounts of the Power. A weak channeler, without such a reinforced conduit, cannot physiologically channel as much of the Power as is provided by the Choedan Kal, even though this may be less than what a stronger channeler can hold with the Choedan Kal based on the idea that angreal/sa'angreal act exponentially. This idea of conduit strength may also be partially explained by the fact that weaker channelers tire more easily than stronger channelers, and why weaker channelers tire even more quickly when using angreal/sa'angreal, in comparison to someone like Rand.