Thanks for your response, that does help.  From what I found - or didn't find, rather - it didn't seem to me like it would be a huge performance impact, either.  A full-scale test would perhaps be the only way for me to learn for sure, but that may not be worth the effort.  The flat file structure is historical now, a new process lands these files in a subfolder structure.

That said, I am still interested if someone else comes across this and can shed any more light on the potential performance impacts of flat-vs-hierarchical source file folder structures with auto loader ingestion.