In addition to 8 GB. being enough RAM for most folks, 16 GB. of RAM won't necessarily improve performance and more importantly adding RAM, even of the identical part number is not guaranteed to function properly - as many people have learned the hard way. If you decide to upgrade you should buy ONE matched, tested RAM Kit, not add another RAM kit or more of the same DIMMs.
It is also true that most mobos default the RAM memory to the CPU's default RAM speed when all RAM slots are filled. There is little performance gain from OC'ing DDR3 RAM due to it's higher frequency and bandwidth over DDR2/DDR.