I ended up trying this anyway, only because I bricked my 980Ti w/a bad BIOS flash.
I have four games that used physx (Fallout 4, COD:Ghosts, Metro: LL Redux and Unturned). I don't know what Unturned gets out of physx.
In my dedicated GPU benchmarking for Metro: LL Redux using the 780 alongside the 980Ti, the minimum frame rates actually went down and the maximums went up while the frame rate variation was clearly reduced over using the CPU for physx calcs.
I have read that some games disable advanced physx effects when there isn't a GPU dedicated to physx.
I wonder if fleX physics (which allow for much more complex particle/fluid based physics simulations) will make dedicated GPU's more worthwhile as PPU's?