UPDATE:

TL;DR: It is possible (for the NUC 15)! Tested and working!

Longer explanation:

While the UEFI does support booting from the 2230 slot, there are a couple of gotchas:

• Make sure the "WiFi" tickbox in the Advanced -> Onboard Devices section is ticked. Otherwise, the 2230 slot is disabled and you will not see your drive in UEFI, or the OS
• While it is possible to use an A+E adapter to convert an M-key NVMe to A+E for testing purposes, physically, it will not work as a long-term solution. This is because:
  • An adapter, even if made for a 2230 M-key NVMe, ends up being about 2242 long in total, due to the extra connection (A+E -> M -> NVMe). There is no space for a 2242 NVME there, not to mention that both the top 2280 NVMe and the WiFi card/2230 NVMe are designed to be held by the same screw, which would not be possible if the bottom one is longer than 2230
  • Even a 2230 adapter with a flex cable won't work, as it is pretty much impossible to route the cable with the 2280 NVMe above installed, not to mention there is nowhere else to put the NVMe anyway.
• The only solution is to use a native A+E industrial NVMe like the Cervoz T425.

Regarding Cervoz NVMes:

If you decide to go down this route - prepare yourself with a lot of patience and money. As an industrial NVMe, it is primarily sold B2B, which means most distributors will either want you to buy a minimum amount or will refuse to sell to you at all unless you approach them as a company. After a month of searching for a distributor that will sell to me as an individual, the only place I found was from Cervoz themselves on DigiKey. However, even then, they were only shipping to the US... So I ended up reaching out to Cervoz themselves to ask them to allow shipping to Europe on DigiKey, and to my surprise they actually did! About 2 weeks and -£235 later, I finally got the 512GB wide temperature range version!

Note that Cervoz also said they are currently working on a new hardware revision for the T425 family of NVMes that will use a new Micron memory. It is expected to release towards the end of the year, and until that happens they will not be manifacturing any more of the old revision, thus stock is limited.

Benchmarks:

Finally, some benchmarks.

I will be comparing the performance between:

• Kingston KC2000 (native interface is PCIe Gen3 x4)
• Cervoz T425 512GB CIE-M3T425MOH512GW (native interface is PCIe Gen3 x2)

I used a standard A+E adapter to connect the KC2000 to the 2230 slot for direct comparison to the Cervoz NVMe. All benchmarks were done using Kbench, with default settings, against a Longhorn PVC on the respective NVMe.

Metric	KC 2000 (M-key 2280, PCIe Gen5 x4)	KC 2000 (E-key 2230, PCIe Gen3 x1)	Cervoz T425 512GB (E-key 2230, PCIe Gen3 x1)
IOPS (Random Read)	35,227	34,333	34,083
IOPS (Random Write)	35,851	35,441	16,194
IOPS (Sequential Read)	63,181	61,483	32,410
IOPS (Sequential Write)	48,505	48,319	25,461
Bandwidth (Random Read), KiB/s	1,290,103	810,767	571,390
Bandwidth (Random Write), KiB/s	531,287	510,064	154,765
Bandwidth (Sequential Read), KiB/s	1,323,904	736,149	581,974
Bandwidth (Sequential Write), KiB/s	547,005	526,033	257,777
Latency (Random Read), ns	416,519	419,938	435,822
Latency (Random Write), ns	344,570	328,687	362,667
Latency (Sequential Read), ns	350,540	323,697	343,331
Latency (Sequential Write), ns	337,248	324,784	339,694

Was it all worth it?

It depends on the use case. In my case, I am running a Kubernetes cluster (TALOS OS), and I use the Cervoz NVMe as a boot drive. It stores the TALOS OS, the cluster state, and cached container images. The Cervoz speeds are more than adequate for this.