I have two of these on my Crosshair Hero VIII, they do get warm, because they really are super fast. I do not use the built in Asus heatsinks with them, the mother board heatsinks are generally useless for any drive with decent performance. I replaced two older 1TB SN700 Drives and even they went over 55C when gaming most of the time with the Asus heat sinks.. I bought two of these, for $11.99 each
These heatsinks offer full coverage, front and back, with metal and thermal pads and are very compact and do not interfere with any other components. They look great too, come in lots of different colors, and are dirt cheap. With these full coverage heatsinks they run about 35C under normal Windows use, and I have not seen them break 46C under load. I did not even attempt to try and run them with the stock Asus cover plates,, because that is mostly what they do, cover them, really not a true heatsink when there is an decent amount of heat on the backside of any performance NVME drive that just gets trapped in there and warms up your motherboard underneath too. I
I get amazing performance with these drives, IMO, the SN850 is really the only Gen 4 drive worth spending money one they are that quick. It is also the fastest NVME available at the moment, even beating out Samsung's new 980 Pro. I get slightly over the tested speeds (from another review site). About 7080.00 mb/s read and 5249.80 mb/s write average, when you enable "game mode" in the WD SSD Toolbox. I see noticeable load time decreases in a number of games I play, and even booting Windows. I also work with a large uncompressed audio library, and an even bigger photo library and see gains there as well. Worth every penny if you have a Gen 4 motherboard, or plan to upgrade to one in the near future.
I own the Crosshair VI, VII, and VIII, I actively use all 3 every day. The VIII is on my Test Bench In an Open Air Setup with a 5900x, the VII is in my Main System running a 5900x (support came way sooner then most of us expected, I was able to get my hands on a Beta Bios with support for 5000 cpus less than a week after launch, which is the only reason I havent moved the VIII over to my main Rig yet, there is still more testing to be done in a controlled environment, and PCIE4 is crucial to my current testing), and the VI is in my Homebrew Automation/VDI/Game Streaming Server, running a 3950x.
They all include some form of PCIE "cooling", that I agree is not ideal. Particularly on the VI, that tiny little slab of Aluminium they give you that is not finned in any way, barely does anything. But on all 3 of those ASUS boards they do at least all come ready with Thermal Pads.
The truth about NVMe SSDs is, you actually want the Flash to be hot, it actually prolongs its life to run at the higher operating temps, so its not actually the flash that is throttling you. It is ONLY the Controller on an SSD that needs to perform under a specific operating temperature. The Temperature at which it starts to throttle varies from drive to drive, but it is safe and neccesary for long term heavy storage workloads to adequately cool the controller, and under 75c is a common controller temp to shoot for to avoid throttling. And again, while I agree that something like an aftermarket EK Finned M.2 Heatsink would definitely give you lower operating Temperatures under full load, it does turn out that even the crappy slab of aluminum on the Crosshair VI hero is enough to keep most NVMe Controllers under throttle territory.
The trick is, that you only want to cool the controller, so even though they always include (usually just one) full M.2 Sized thermal pad, you want to cut that Thermal Pad down to the size of the controller and only apply it to the controller, and let the Nand operate at its hotter temp. On drives like the Samsung Drive, they will give you 2 Temperatures to Monitor, its important to figure out which is the controller and which is the Flash. Even under longer sustained workloads, it is totally normal and within spec to see the flash operate at temperatures exceeding 80c to 85c, its only the controller you want to see keep below 75c (or the specific throttle temperature of yoir specific drives controller, which can somtimes be lower or higher then 75c). Also another trick to help cool the controller is in addition to placing a thermal pad only over the controller on the top, is to use a full size thermal pad for the length of the entire M.2 on the backside of the drive, to sink the heat of the M.2 PCB into the Motherboard PCB or if your heatsink covers the backside of the M.2, then sink the full backside to that. That actually helps a lot in pulling the build up heat from the controller and both spreading it to the flash and eventually to the heatsink/mobo where airflow in your case will eventually dissipate that heat.
I have done a lot of testing on this and have a lot of data to back this up, as well as old school reviewers like Allyn Malventano at PC Perspective (now retired), and this is definitely the way to go when it comes to cooling your NVMe, but also making sure that you are preserving the longevity of its life by allowing the flash itself to remain hot, while cooling the thing that actually needs cooling.
Its for these reasons, that those largely anemic Mobo Heatsinks are actually more then enough yo spread the heat of the controller and do its job, its just really unfortunate that neither the manufacturers themselves nor the reviewers of these products, educate their customers/audience on this best practice. Obviously I would not just let me, some random internet commentor dictate this to you and take it as fact, I would encourage you to go out and look this up yourself, The aforemention Allyn is a great place to start, although he is no longer reviewing drives, there is plenty of archived content on the PC Perspective Website that he did these reviews for, that can educate you on this, and why it's important to allow Nand to operate at its hotter temps.
The PCIe 4 drives do get even hotter than 3.0 drives, and these controllers really do throttle hard if you do not keep them under around 75c durint those longer storage workloads, but I know for a fact, that sinked properly, the Asus C8H Included M.2 Heat "sink" is enough to keep the temperature below throttle territory, assuming you have a room ambient that does not exceed 30c, a Case Temperature that does not exceed 45c, and decent airflow in the case that travels over the anemic "heatsinks", provided you sink just the contoller to the Heatsink on top, and the entire bottom sinked to the mobo, which is how I do my testing on this very topic. (My company provides the systems/maintenance for specialized fields so its my job to test our build outs for the clients use case, and a big part of that is making sure that the hardware can operate withing spec in their operating environments, so storage cooling is a component in this, which how I became to learn so much on this topic, although there is obviously still so much more to learn!)
Anyways, TLDR: Mobo M.2 "heatsinks" that actually include a thermal pad is fine as long as its actually Metal (Aluminimum or Copper/Nickel Plated Copper) and includes at least one thermal pad. Its also best practice for the longevity of the drive to cut that thermal pad to the size of just the SSD controller and sink only that on the top to the "Heatsink". If its a 1 Sided SSD with no flash on the bottom, then its also a good idea to use the rest of the thermal pad (if its thick enough to make contact) to sink the backside of the SSD to the Motherboard or backside of Heatsink. Flash Likes to be hot, so its best not to cool the flash.
(Obviously there is nothing wrong with buying an even better proper M.2 Heatsink and use that in leiu of the included one, with the same principle instead, especially if you have determined that your sustained storage heavy workload throttles on the stock heatsink.)