Past 1600MHz, there's usually not a real-world difference in RAM speeds unless you're gaming on integrated graphics. Especially given how expensive RAM is right now, there's no reason you need to get any memory faster than that.
8GB is enough. Game usually only use 2-3GB at most. Which leaves you with a couple GB of headroom. 12GBs or more is generally useful only if you use memory-intensive programs, like some kind of media editing.
I don't know much about the Killer NIC. Based on reading reviews, some people have problems with connectivity or higher ping, but I'll let someone else answer as I don't know for myself.
Not much, unless you're using a CPU with high-end integrated graphics(e.g. an APU or a chip with Iris graphics).
8GB should be enough, but if you're not satisfied, you can always get a cheapo 4GB kit and throw it in for 12GB, which should be enough for anything reasonable.
They're almost pure marketing. Remember that with motherboards, you pay for premium features and not premium performance.