Physx isn't really worth it anyway. I do not miss it at all because most games don't use it.
If Nvidia made it possible with a separate app that allowed the use of the Nvidia cards for physx along with AMD cards for video, then I would do it.
What would probably be the minimum NVidia card to use as PhysX processor? I got an old 8800gts 320mb laying around, would use it conjunction with incoming r9 290 if it would do some good.