I don't know that that is true Trog... rFactor was built for a single thread, before dual or quads and the game engine is actually even older than that. They actually used the -FullProc to turn off a thread. Then it was reversed to turn on threads for the single thread software. So there could be some key to it. That's why i want to see the actual Microsoft writeup and description the actual tool and not the proprietary named command.
I saw a 100% increase in FPS with it on as opposed to off. as can be seen in the following screenshots. So there definitely is something to be had in there.
nofullproc.jpg
fps.jpg