well I all ways been told 2x8GB is less stress on the MC (memory controller) and can be ever so slightly better performance wise , but cant prove it
one of my AM3+ builds [asus] would not take any kit of 8gb sticks [@1600] with a 8350 or 6100 cpu ?? memory kits that failed in that worked excellent in a intel build just set the xmp and run stable .. it was a headache to make that run stable with anything cpu or memory .
my buddy put 4x8 1866 in his giga 990fx with a 8150 and worked fine but at loose timings kinda like I showed above [1735@9-10-10 - 9] that was all hen could squeeze out of them stable.
that when we started saying as a joke ''if its AMD its 1333 '' that was one platform I ever had so much headaches and cost me more money trying parts to make them satisfactory . cut my losses and jumped to intel thing is we talked about how our old AMD builds we never had issues just build them up set the bios and run the heck out of them . AM3+ =nope
may just come down to having to underclock from 1866 to the best you can get and be 24/7 stable may even be stuck running at that default 1600 as best it can do ??
like here seems 2 sticks of 1866 worked fine but 4 failed ??
''Quick update: I managed to get 4 sticks running at 1600. All I did was enable the XMP profile but lower the multiplier to 1600 instead of 1866.''
So now its running @ 1600CL8
http://www.tomshardware.com/answers/id-2177084/issue-ram-970a-ud3p-failing-boot.html