Based on cspear's graph, as posted above by Miss Nancy, an IC value above about 60 or 70 is not likely to render any faster than IC=100... am I interpreting that correctly?

I seem to be getting more black artifacts with higher Pixel sample / shading rate / IDL sample values, if I leave my IC at 20... and whilst there was a little improvement with the original shading distortions under the cushion of the white chair in my render, this approach hasn't resolved the issue altogether... and more black hole type artifacts are now appearing, scattered more randomly around the image.

So I may have to just bite the bullet and try again with IC at 100 I guess. Or perhaps look at the geometry of the chair mesh, to try adjusting what I think are the problematic polygons. I might try just subdividing it and see if that reduces or impacts the original distorted shadows at all...

 

Can adding AO nodes to your shaders, with "Evaluate in IDL" ticked, help to compensate at all for the loss of small scale occlusion when IC is set low??

Many thanks