btw the SKU number you provided CMX8GX3M2A1600C9 does indeed point to an 8Gb (2x4Gb DIMM) kit, on the Corsair web page.

https://www.corsair.com/uk/en/Categories/Products/Memory/XMS3-%E2%80%94-8GB-%282x4GB%29-DDR3-1600MHz-C9-Memory-Kit/p/CMX8GX3M2A1600C9