I have a ASUS Rampage V Extreme Motherboard, with a i7-5930k CPU I built a while back.
It currently has (2) EVGA GTX 980 cards in SLI.  
 
I am upgrading to (2) EVGA 1080 Hybrids in SLI.  My MoBo will do x16 and x16 for the SLI config.
However, I was wondering if it would be better to go with the 1080 SLI + a 980 dedicated to Physx?  (at x16, x8, x8)
 
I can find older posts in forums saying the x16 vs x8 for graphics cards makes no real difference.  I have also seen older (dated) posts showing a slight performance jump in a dedicated Physx card... and with 1080's, it may not even be worthwhile.. but thought I would ask and see if there is any guidance out there.
 
Thanks,
 
Phil 
 
 

Hopefully he has a good reason for a physX card. There still coming out with games that use physX?

A 980 sounds like complete over-kill teamed up with a pair of 1080's just for PhysX work offloading. You must be either really, REALLY into a 
specific game, or a maniacal power hungry gamer with money to burn. Either way, who can blame you for trying ! lol, just post them results. 

 Nice, can't wait to see your results. I'd love to see more PhysX tests done with various combinations of hardware, you don't see enough. 

So, I used the PLA Game Benchmark, , as suggested by Sajin. 
 
2 x GTX 980
Before upgrading my system, I took a benchmark run on my EVGA 980 GTX SLI cards.

  
2 x EVGA 1080 FTW Hybrid
I then replaced the 980 cards with the 2 EVGA 1080 FTW Hybrids I received this week, and reran the benchmark.  In both of these (980/1080) SLI setups, the Physx Settings are using recommended setting (GPU2). 
 

 
2 x EVGA 1080 FTW Hybrid + 1 x EVGA GTX 980
I then added one of my GTX 980 cards back into the mix.  The 1080s are in SLI, and I set the 980 dedicated to PhysX.
 
After setup, I ran the benchmark again, and was amazed at the results... and not just the numbers, the benchmark run-through looked fantastic.
 
While PhysX is not used in many titles, the gains based on this benchmark have convinced me it’s worth keeping one of the 980 cards I was replacing in this upgrade and running it dedicated to that processing.
 
i7-5930k, Haswell-E 6 Core, OC 4.1 GHz, Kraken x61 Water Cooled
Asus Rampage V Extreme
G.Skill 32GB 2666MHz DDR4 15,15,15
Samsung XP941 M.2 512GB PCIe
EVGA 1080 FTW Hybrid SLI
EVGA GTX 980 Gaming w/ACX 2.0
EVGA SuperNOVA 1200 P2 Power Supply
Thermaltake Core X9 Case

Nice results.  Thanks for posting...
 
I have always kept a GPU from the previous generation to use as a PhysX card.  Not only that, but I prefer connecting my 2nd & 3rd displays/TV to that card, leaving just my main display on my primary GPU.

I too saw dramatic score gains in the PLA test using a single 1080 and my 960 for PhysX. In the Batman games and Metro Last Light I saw modest average framerate boosts, but larger gains in minimum framerates (for less stuttering). The problem is that the PLA test isn't a game, so you run it a few times you're basically done with it. I've still never played Batman, and have only played MLL for a couple of days before moving on to other games, none of which (so far) has used PhysX.
 
So the problem isn't that it isn't beneficial in games that use it, it's just that there aren't that many games I'm interested in that actually use it.
 
That being said, I put the 960 back in recently when I began participating in the Folding @ Home project for Team EVGA. If you're interested, your two 1080s and the 980 would put out some killer points per day.

wow! thats saweett!