While working around the ground plane issues I came across this one:
Setting up the custom render mesh for the backdrop is, btw, also not working properly here: Studio.3dm (225.1 KB)
Setting up the custom render mesh for the backdrop is, btw, also not working properly here: Studio.3dm (225.1 KB)
The rendering artifacts you are seeing are not from the ground plane. Your curved surface is rather large. The current setup gives extremely long and thin faces. You need to rebuild the surface to get better rendering resul. I.e. 10x10 rebuild gives nice rendering.
I never realized that an extruded curve “ignores” the maximum edge length when using a custom mesh. But rebuilding it, it works and looks great. 
Thank you, @nathanletwory!
No problem. I was scratching my head there for a while as well. It is a good thing I gradually learn about how Rhino works, as well. Makes it easier to pinpoint weirdnesses 
/Nathan