The difference between single-channel and dual-channel is a few percent. 1600 (12800) is the price/performance sweet spot for Sandy Bridge systems.
As far as your last question: I don't know ... is it? Do you notice a difference with only one stick? Do programs/games load just as fast as they did before, or is it slower? I know I noticed immediately when I went from 8GB to 4GB, but depending on the programs/games you use every day, you may not.
G.Skill has a lifetime warranty. Contact them and RMA the memory. You should at least get back to the 8GB you already paid for.