I just upgraded the memory on my late 2015 iMac from 8GB to 32GB and to future proof myself a little against having to replace them again in a year or so, I bought 4 x 2133 MHz HyperX modules from Kingston. After inserting the new chips, I was somewhat surprised to see: 'Your Mac contains 4 memory slots, each of which accepts a 2133 MHz DDR3 memory module'. I'm not sure whether this means they are actually running at the increased speed, but it would be an interesting fact if they are. PS As a side note: in normal use, the new iMac seems to use around 60% of the power of the late 2014 iMac. Not sure that'll contribute much towards the cost of the next upgrade, but I'm happy with the reduction in power consumption nonetheless!