There isn't much bebifit from overclocking the ram so it's not worth the trouble to fight with it to get it stable at higher speeds. You would see a couple of FPS more at most.
I agree. There isn't much performance gain to be had from overclocking RAM, and if you are already running 1600MHz at 1.65V that should be the top end of your RAM anyway. 1.65 is a high voltage already and if you're not able to boot at any combination at 1.65v then you probably shouldn't go any further.
As stated previously, 1600MHz is more than enough, those higher clocks won't show you any real performance gain.