They TESTED different card configurations and discovered that if an ADDON PHYSX card was too slow, the game actually ran SLOWER.
You'd probably need a GTX660 or greater to prevent a slowdown.
If a game can't maintain 60FPS on HIGHEST SETTINGS with PHYSX enabled then don't use PHYSX. PHYSX is addon eyecandy that often adds little but can seriously drop the frame rate (i.e. Mafia 2.).
Borderlands 2 is one of the exceptions but then you don't require an addon card for that.
Batman Arkham City has serious stuttering issues. The ONLY solution for smooth game play is this:
1) disable DX11 features, and
2) disable PHYSX
Neither feature adds that much most of the time, but the game is far, far smoother when disabled (tested on i7-3770K, Asus 680 TOP, 16GB 2133MHz).