What you are reading is the Qualified Vendor List. That doesn't mean you can't use 2x8. That motherboard has Single Channel and Dual Channel technology. You can basically use any type of DDR3 RAM with that motherboard. Asus, is suggesting you use 4x4 configuration. However, this isn't necessary nor is it the only thing you can do. A 2x8 configuration will work just as a 1x16 configuration will work or a 4x8 set up. How else would the maximum supported memory be 32GB?
Again, you do not need a 4x4 kit. If this is just for gaming, I suggest a 2x4 kit. If you truly want 16GB, a 2x8 kit is what you should buy. You also don't need to buy 1866MHz if you don't want to. There's no gaming performance difference between 1600 and 2800 RAM. If you...