BabaJ

I haven't looked at your scene file;

But for your intersection function did you check to make sure the geometry you are running over isn't including the position you are searching from too?

You may need to use the intersect_all function and take the second item in the returned list.

There are elements of this tutorial which will help to focus in finding where most of your noise is being generated from.
You might find it usefull to go over the whole tutorial. It covers both CPU/GPU.

https://www.sidefx.com/tutorials/refining-karma-renders/ [www.sidefx.com]

Mouse over viewport and press D for 'Display Options: world'.

Under Geometry Tab there is a section for Tessellation settings you can adjust.