Samsung SM 951

A SM951 might get hot being so close to the video card.

Part of the problem is both of the Samsung M.2 SSDs (XP941) run warm when performing IO tasks over a long period of time without any breaks, such as while being benchmark tested. They don't have the metal case as standard SATA drives do which act as a heat sink.

One thing that will help when using a SM951 is the way ASRock designed the Ultra M.2 connector. You can see the metal stand off that holds the M.2 drive above the board by about 2cm, and the M.2 slot is also located the same height above the board. That will allow air circulation around the M.2 SSD. But you will need to try to get air circulating in that area to help cool the M.2 SSD.

The design of the cooler of your video cards will also be a factor in how hot a SM951 would be. Some video cards push most of their heat out of the back of the card, by the cable connectors. Others just push their heated air into the PC case. That kind of video card will be worse when trying to keep an M.2 SSD cooler.

While some of this sounds bad, the real world reality of using a SM951 is a different story.  Samsung is primarily selling these SSDs for use in laptop PCs that don't have good cooling at all compared to desktop PC cases. Laptop PCs are also internally much warmer than desktop PCs. If the SM951 and XP941 were having overheating issues, we would be seeing reports of that in PC hardware forums, and in reviews of these SSDs. I haven't seen any reviews that said these SSDs were having heat issues in normal use, so I doubt you will.