whether or not it runs at x8 or x16 makes no difference. What matters is if you will benifit from a physx dedicated gpu. do you run games well optimized for physX? If you are running a 780 it is doubtful that you will get any noticeable improvement in 90% of games. The only way to really know is try it though.
You will see some benefit in GPU accelerated PhysX games, but only in those games. As a rule of thumb (varies widely) you should see roughly a 25% performance increase. Dropping the PCIe bandwidth will have no noticeable effect on your performance.
The big issues are:
1) Is your performance with a single GTX 780 sufficient (FPS near your monitor refresh rate)? Do you NEED more performance in GPU accelerated PhysX games?
2) Is the performance benefit enough to justify the additional heat and power consumption you will experience with the extra card installed?
3) Is your power supply enough to handle two high-end cards?
4) Do you play GPU accelerated PhysX games enough to warrant having a dedicated PhysX card?
1) I don't think so now that I tested it. I left it in there when I got my new card because 3) my power supply was beefy enough.
2) Probably not.
4) I have, and it has helped when I was playing Arkham Origins, but since I don't play that anymore, I might as well take it out.
I asked because when I was having poor performance in Wildstar they told me it was because my main card was only operating at 8x, which I have now found it not correct, so thanks for that too!