first of all you need to understand about physx. off loading physx processing onto gpu will slow down the performance. but that's only true if the calculation are suppose to be done on gpu and not cpu. this only related for games with gpu accelerated physics. with amd gpu those physics effect are disabled. for new version of physx some of the complex calculation can be done on cpu (borderlands 2) but it will lower the frame rates.
currently nvidia are working to make physx to be more cpu friendly