I recently bought an ASUS N550J with
4 GB Dedicated VRam GTX 850m
When I use blender the GPU renderer only renders one tile at a time and the time taken by the GPU is more or less equal to the time taken by the CPU. Sometimes it takes more than the CPU.
I tried all of your suggestions and it works great! It took approx. 3 minutes on CPU and only 1.5 minutes on GPU. I was using the BMW1M Blender Benchmark Model.