Blender 2.79b
Best time is 4m 42s with CPU. I was very surprised than GPU was slower 
GPU NVIDIA GTX 660M
CPU INTEL I7 3630QM 2.4 Ghz
GPU 64 64 9:12
CUDA 128 128 8:25
256 256 8:20
356 356 8:42
512 512 9:23
CPU 8 8 4:56
16 16 4:52
32 32 4:55
64 64 5:01
128 128 5:39