M.2 SSD Installation iMac 27 "5K Retina

"They just don't seem to work very well even with the adapters. I would stay with either an original Apple stick (high dollar) or go with an OWC stick (I don't think they make one yet for this model as they only list these upgrades https://eshop.macsales.com/upgrades/imac...).

It's also very difficult to install: iMac Intel 27"" Retina 5K Display Blade SSD Replacement

Personally I'd add just a standard SSD drive like this: How to Add An SSD To The 27"" iMac (Late 2015)"

"I replaced my stick and its not a change at all I get the 1 Terabyte but the speed is about the same just volume gain thats ok. I never had a fusion drive so I did get to put in a 500 G where there was none, thats a gain to and faster than external by just a bit. The NVME is better off on the outside if you have a fusion drive inside its a easy change up.

If there is going to be a Firmware update to stop the bottlenecking on the 15,1 27” iMac then we will see a gain.

Personally I thought the process of putting in a new SSD was very easy I had to add the cable and heat sensor so if I was going all the way I threw in the new stick too. Yes, you're going all the way in, so really if you follow the fix it instructions its easy. I would do it again in a minute. Biggest worries don’t drop the screen."