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?
So what PhysX games do you play?http://www.physxinfo.com/