The G3258 cannot run the RAM at DDR3-1600; DDR3-1400 is indeed as fast as it gets.
A higher setting than 4.2 is available, but 4.4 appears in a hazardous-looking red font. Being somewhat risk-averse, I just left it at 4.2. Eric Van der Linden is looking at the "Z" overclocking boards, and is pushing higher clocks.
If you want more than 16GB, you'll need four DIMM slots. Or, you get to start with 2x4GB and add another 2x4GB later to get 16GB without having to discard your prior RAM.
If you want to overclock a "K" chip, this won't be the board you buy. Otherwise, it is very hard to beat this one if you might want any of its other features, like RAID.
I did notice that one chart got entered as "Haven" rather than "Heaven."