Borderlands 2 GPU & CPU Performance Test - TechSpot Reviews.
I finally got into a big enough fight last night where I wasn't getting a low frame rate so much as the game was pausing for like a half second here and there. Time to turn down physx from high, I guess.
Others report that you may want to set your Nvidia Control Panel to force Physx to your GPU. It defaults to auto and some seem to pick the CPU instead.