OWC is the first memory vendor I have found to date to carry the 1866MHZ ram upgrades you need to max out the RAM on the Late 2015 27 inch iMacs:
Currently, they stock the following:
This means that you have a few choices for upgrade configurations using these 3 different memory upgrade sizes combined with the 4 slots available and if you really need it you can spend more than $1000 on ram for new 27 inch iMac models. I didn’t think that was possible in this day and age still.
Your options for ram upgrades are:
8GB Memory upgrade kits
16GB Memory upgrade kits
32GB memory upgrade kits
64GB memory upgrade kits
Remember these new late 2015 iMacs don’t take the same ram as the older models so the pricing is different for the newer and more expensive RAM. You need 1866MHz PC3-14900 SO-DIMMS for Apple’s latest Macs.