It depends on the GPU you would be adding. Some cards compute Physx slower than your 980, so it would reduce performance. That being said, almost any card would be wasted energy and heat, as the 980 is more than capable.
yeh but with new direct x 12 on the horizon u can stick any two cards together and they will stack even ram.
Then the one wouldnt be processing Physx for the first anymore. Plus a setup like that is bound to have more issues than good. Different architectures with varying memory sizes across different sizes buses dosent seems very efficient.
There is also the fact the 980 shouldnt need any help to begin with.
yeh i know its powerfull enough nothing wrong with more also new direct x 12 will read multiple cards as one thats why there wont be any complications and why they stack vram etc u can even put amd with an nvidia card aswell