Multi-GPU Scaling
Just like any other Multi GPU solution, the performance gains increase with resolution. One reason is that at lower resolutions it is more difficult to split the work load between GPUs and it is also possible the the rendering is CPU limited. At lower resolutions some benchmarks can't just render any faster, no matter the graphics power, because they have to wait for some data to be calculated on the CPU every single frame.
Please also note that the 4-GPU setup failed to complete several benchmarks at 2560x1600 which results in a lower score.