From what I remember from prior testing, dedicated PhysX needs a card that was within about 20% lower performance (of the primary video card) OR BETTER for an improvement. (..it also needs to be Nvidia with Nvidia, not other cards.) Also, the improvement seemed to "top-out" for an equal card of around a 15% performance boost, with lower performance cards (again, 20% drop) often providing about 10% improvement. (..it's been a long time since I looked at this though.)
If we apply the guess above, a standard 980 GTX (as a PhysX dedicated card) should provide almost a 10% improvement to a 1070 GTX. A 970 GTX might still provide some improvement, and I think it's likely that anything lower than that (960 GTX) would start to degrade performance. Might be wrong though, and if you've got the dissimilar cards you can always try it out.
Note that as long as the drivers (and game) support DX12's video-card "teaming", that the benefit should be much higher. Nvidia has huge resources when it comes to improving drivers, so my guess is that it won't be long before Ashes of the Singularity (a DX12 game) becomes a game that Nvidia cards see major improvements with adding video cards for full processing as opposed to a dedicated PhysX card.
Still, I can see people doing this for SLI configurations and games that aren't DX12 with supported drivers for "teaming". EX. 2x 1070 in SLI and along with a 980ti or Titan X (as a dedicated PhysX card) for everything BUT DX12 games that support "teaming" - and for those: "teaming" alone (or SLI and "teaming"). Provided the driver's are up to it: that should provide a lot of performance.
