Memory speed does not really influence the actual performance of your system. Practically all performance tests will show similar results for DDR3 1066, 1333, 1600 or 1866.
Since June 2016 I have a late 2015 Retina iMac with 64GB OWC RAM, very unstable. Replaced RAM, back to Genius Bar, etc.
Memtest86 appeared to pass everything, but only if I limited the number of CPUs to 1. I've had several occasions where the keyboard was not recognised on boot.
If I do the mersenne prime tests, I get hardware failures: http://www.mersenneforum.org/showpost.php?p=449730&postcount=588
The two responses I received were to update the bios and to suspect the RAM.
Based on this and amazing support from VMware we have been attempting to get Apple to recognise that new microcode for the Skylake CPU is needed which Intel released last year.
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Memory speed does not really influence the actual performance of your system. Practically all performance tests will show similar results for DDR3 1066, 1333, 1600 or 1866.
This is an interesting video about it showing some performance-charts from about minute 2:00 of the video: https://www.youtube.com/watch?v=dWgzA2C61z4
On Amazon, you can now buy PACCOM 64GB memory kits for the new 2015 iMac 27" at only $729 with 1600 speed. That's a deal!
Regards,
TW