I was wrong. It's not working beautifully. I only tested simple flags in Mirror's Edge. There's an area just after meeting up with Kate for the first time (when Robert Pope was found dead in his office) where when you're running out of Pope's office the cops start shooting at you through the windows. The PhysX processing on this becomes painfully slow and the game crawls to what feels like 5-10 FPS.
Unfortunately, the NVIDIA Control Panel cannot be opened, so I don't even know if my GTX 580 is doing the processing.
This is making me think that I should return the R9 290 and just save up for a GTX 780.