corsair RAM for IMAC 2017 5K

Your absolutely right. From what I can tell the Dominator Platinum is not certified to work with Mac. My assumption was the CDP was in fact a Mac tested product since Corsair does make Mac compatible RAM. So they make Mac RAM, just not for that line.

If this is the RAM the OP is interested in they need to verify that it will work this this model Mac from the manufacturer. I should have been clearer that Mac certified RAM and RAM that may be the same form factor, latency, speed and voltage may not necessarily work in a Mac if it was not clearly stated as doing so.\

Use the Mac operating system that came with your Mac, or a compatible newer version - Apple Support

this is correct to a fault.

I had downgraded a number of iMacs that originally came with one OS but were upgraded after it was superseded. The downgrade came out of necessity and had a sr. dev from Apple go over the pitfalls - apparently there were none, and the warranty and apple care and support remain intact so as long as it can accommodate an early iteration of OS X you are completely in your rights to do so if you own it - however if you owned Sierra then Apple has apparently removed it from the App Store and you will be taken straight to High Sierra when you check your purchase history.

so long Sierra, good thing I keep archives.

1. Your much better off buying RAM from vendors that actually tests and specializes in Mac memory,

like > Apple Mac Memory/ RAM Upgrades from MacSales.com

or > Crucial.com

2. No

see > Use the Mac operating system that came with your Mac, or a compatible newer version - Apple Support

you can downgrade provided the line of your mac was released at the time of a given OS X, however the 2017 is not eligible for OS 10.11; the release of that system was on June 5, 2017 when Sierra was public and honestly Sierra is not going to be much different than 10.13 if thats what comes on the device. If you do decide to downgrade to 10.12 your device will still be in warranty.

If the RAM is compatible and the same form factor you should have no cons.

260 Pin PC4-19200 DDR4 2.4GHz.

you may see a little bit of a slug during boot and initial mount of the OS, but nothing more than that - and it may be imperceptible.

More RAM is generally far more Pro than Con especially if you are doing 3D or Video Editing.

Good work. 😉

Yes, the Spec's and Compatibility are far more important than rushing or trying to save a few bucks.

FWIW I can not bad mouth Corsair, because I had it in my Early 2006 iMac for years and it never gave me any problem.

Every PC build I've had used Corsair. They also make great cases and PSU's - just not for Macs!