I've personally looked through different forums and found mixed opinions on this topic. I own an msi GTX 660, a great card that replaced a GTx 460 array. At this point, I still have the 460's as they are currently selling in eBay. I hooked up a 460 with the 660 and dedicated it to physx. I tested it on Batman arkham city, being that it is the game I have that gets bogged down by physx being enabled. It goes without saying, this game has issues with stability, the dedicated 460 gave me a performance gain of about 5 FPS. On normal physx setting and on high. Mixing up the rest of the settings, turning off dx11, etc... I saw no gain over a single 660. In fact, I saw a performance decrease in games like Skyrim, and Max Payne. I noticed stuttering and frame drops for the first time. Also, the 660 would run about 10 degrees hotter at load and stayed hotter longer. The 460 would show about a 7% power load at its max, that being said, would probably translate to about a 3% load relief for the 660.
I tell you, in my opinion, the extra heat and the lack of Any significant gain, made using my old card for dedicated physx, is not worth it. Even if you have an extra lying around, it isn't worth it. Sell your extra.