JSung,
AHCI: I'm using a Samsung SM951 in a z420 and z620, These are both 256GB AHCI versions, the z420, mounted on a Lycom DT-120 M.2 to PCIe adapter card and the z620 being an HP z620 Z Turbo Drive which has an SM951 as the drive .
The SM951 AHCI works very well in both systems, the Passmark Disk rating being 11559 for the z420 and 12915 in the z620. The Z Turbo Drive specification shows a bit better performance than the SM951 used with a third-party adapter card probably as the Z Turbo is on a proprietary card developed for HP systems. Earlier on, M.2 was mentioned as running very hot and the Z Turbo has a substantial heatsink on thermal pads that cover the memory chips.
NVMe: Among the Passmark Performance Test baselines, there are two listings for a z620 system (2X E5-2620 / GTX 970) shown as using a Samsung 950 Pro NVME 256GB drive. I think it's the same system with 16GB RAM (Passmark Disk = 10085) and then with 64GB (= 12650). If more RAM only makes a substantial improvment in Disk speed, that's an interesting result.
As for the z420, there is an E5-1650 v2 system with a Samsung 950 Pro NVMe 256GB , scoring 15187.
The z820 uses a different BIOS, but there is one system -also listed twice- shown as having a Samsung SM951 NVMe 512GB with the Disk = 10383 and 12141.
I'm not sure of the reason that there are a couple of successful uses of NVMe M.2, and more 950 Pro than SM91 on zX20 systems, but it may be that the firmware in the newer 950 M.2 or system BIOS and chipset drivers are working towards a greater compatibility with older systems. If it's possible that the zX20 systems' can have BIOS / firmware developed that allows the use of NVMe drives that would of course expand the selection of drives.
M.2 makes impressive numbers, but in my use has made little operational difference as compared to the Intel 730 480GB that was the main drive in the z420 (Passmark = 4764). The startup and file loading iand saving is apparently limited by the CPU (E5-1660 v2 in the z420 and 2X E5-2690 in the z620) so most functions seems very similar. When making transfers, the limitation might be the other drive being able to keep up in read and write speeds. As I use conventional mech'l drives for backup, the M.2 advatage is completely lost. However, I imagine that in very heavy disk-intensive uses, the difference would be more apparent.
Cheers,
BambiBoomZ