It's not a question of whether a game supports PhysX, but whether a game uses it. From the little bit of research that I just performed, I found that GTA V does use PhysX, but it sounds like it's not really extensive. To see for yourself, I recommend searching the internet for things like "GTA V PhysX" or "does GTA V use PhysX", or you might even benefit from similar YouTube searches.
Anyway, if you were to see a performance benefit by dedicating the GTX 480 to PhysX (I wouldn't know), then it would probably be a tiny amount because the GTX 780 is already very powerful. Fortunately, a video card being dedicated to just PhysX won't really get very warm at all because PhysX is very easy for these modern GeForce GPUs. So, the common heat problem of the GTX 480 wouldn't really be seen since it's not being asked to do any 3D rendering.