It's not really a chip - it's a wrapper to implement the PhysX API through CUDA, and uses the same processor used for graphics - which is made up of many "stream processors", which are very specialized and mostly geared toward vector operations, which they are quite fast at. (A modern nVidia GPU can completely outclass a top of the line x86/64 CPU at things like Folding@Home).
Vector processing is useful for physics - but I'm not sure they are precise enough for Orbiters usage.
In any case, from my experience profiling other apps that have similar physics to orbiter, little time is actually spent in the physics calculations. The physics load of a modern FPS game is much higher than that of orbiter - many, many more objects, collision, etc., and they generally do well enough without needing PhysX hardware.