I have a fairly new, supped-up gaming rig:
CPU: i7-6800K @ 3.4GHz - 6 core, 32 GB RAM
GPU: 2x Dual GTX1080 - 8GB, 2560 CUDA cores
Windows 10 v10.0.14393
Blender 2.78c
Here are my benchmark numbers for the BMW27 test (1920x1080@50%):
GPU:
240x180 0:33.84
480x180 0:31.83
480x270 0:31.13
480x540 0:34.95
CPU:
128x128 1:55.78
64x64 1:56.80
32x32 1:48.42
16x16 1:47.45
Clearly, the dual GPU beats the CPU (even though CPU is running 12 concurrent threads)
The best time with the GPU was 31.13 seconds @ 480x270 tiles (4 squares)
Classroom (1920x1080@100%)
GPU:
480x270 3:52.08
480x180 3:47.95
Pretty cool, eh? 