I see there yet another conversation about Async.
I have a little experience with it, Nvidia does have ASync, i know that because i use it in Volxel Global illumination.
The pre-render is calculated on the CPU A-Synchronously, The GPU then projects Shadow Maps for Occlusion and Lighting through Ray-Tracing.
It Works but the result does have a heavy hit on the GPU, about 2ms per frame worth of render cost on a clocked GTX 970, significantly more if you have a lot of shaded poly's.
Let me tell you Async on this current generation of GPU's is not great, its not something these current GPU's are designed for, the performance hit is significant but even if you have enough power to drive through it for high Frame Rates the experience is not smooth.
Being capable of it is one thing but doing it well is quite another.
In my experience and opinion we may have to wait for Pascal to get better A-Sync hardware before Ray Tracing GI becomes completely viable, its too much to ask at the moment, it can be tweaked to be less demanding but its at the cost of its effects, and whats the point in that?