To test the system we used blender 2.75 x64 release. The cycels renderer utilized all four gpus.
Both scenes downloaded from blenderartists.org The updated file rendered in 00:37,38 sec. The older version of benchmark file BMW1M rendered in 18 seconds. Cycles with blender shows significant performance increase on newer series of top nVidia GPUs (series 9xx, 7xx) compared to old ones (5xx and 6xx).