We are considering Mantle at the moment. It looks really interesting

We don't use PhysX but our physics solution is highly multi-threaded. Many of the 'scalable' GPU style physics behaviors are also do-able as particle effects which use the G buffer to compute physical interactions with the world.

Different platforms afford different advantages. Modern PCs with high-spec graphics cards cannot be beaten for shear raw power. What this translates into are higher resolutions and frame rates, which are always good. Also we can afford to scale assets (geometric, textural, animation, etc.) to take advantage of the extra fidelity. With higher resolution and more raw power available tessellation can occur earlier. Shadow algorithms can also be more realistic.

Many full screen effects can be refined due to the available power, for example ambient occlusion, anti-aliasing techniques, motion blur and depth of field and high resolution multi-monitor support can only be achieved by utilizing the GPU power available on the modern PC platform. We're also looking at support for 4k resolution for the best fidelity possible.