I think this article may answer some of the questions you posed:
http://helpcenter.graphisoft.com/technotes/setup/software-technologies/multiprocessing-and-archicad/
It is a bit older article but still has some value I believe. But for what it is worth, Graphisoft now recommend CPUs with 4 or more cores, because it will use all those cores:
http://www.graphisoft.com/support/system_requirements/AC18/index.html
There are now many areas, like generating Priority Based Connections (PBC), or generating Section/Elevation or Layout data, which benefit from more cores. Also, CineRender, obviously, uses all the CPU cores it can.
Also, AC17 introduced background processing:
http://helpcenter.graphisoft.com/technotes/archicad-versions/archicad-17/
ArchiCAD uses background operations to calculate the precise geometry of construction elements which intersect or collide with each other. Background operations means that you can, for example, work on a floor plan view while the application calculates the geometry of elements not visible in the current view – without interrupting your workflow. Benefit: when you switch to another model view, the result will be displayed much faster.
So it seems more cores do improve performance, but I am not aware of exact figure of just by how much.
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