Well I've just done some basic benchmarking using Batman Arkham City, Arkham Origins and Metro 2033|
In both Batman games, there were drops below 30FPS when the GTX570 was disabled and high PhysX enabled. When I re-enabled the card, the FPS remained almost steadily at 60FPS at those same moments.
In Metro, it appeared to only make 1-5 FPS difference, suggesting that whatever was causing the FPS to dip heavily at times was due to some other setting.
I suppose I'll keep using it for PhysX enabled titles.
I've a new motherboard on the way, which I think has it's PCIE slots a little further apart, so it shouldn't be much hassle to just slot it in when I'm playing a PhysX title, and leave it out when it's useless.