Calaudude,
I've used relatively few SSD's, but the best performance by far have been the two M.2 using the Samsung SM951 256GB AHCI.
z420: The z420 (Xeon E5-1660 v2 .32GB / Quadro K4200) that I use for 3D modeling has an SM951 mounted on a Lycom DT-120 M.2 to PCIe adapter card in PCIe Slot 1 and includes the OS /Programs, and active projects. That system previously used an Intel 730 480GB. The Intel 730 is quite fast but to summarize using Passmark Disk results in the z420: Intel 730 = 4760, and Samsung SM951 AHCI = 11591. To put his into further context, the Intel 730 cost $290 or $.61 /GB- remember it's 480GB instead of 256GB. and the SM951 was $157 + the adapter card at $22 which is about $.70 /GB. In a rough cost/ performance: the Intel 730 cost $.15 per Passmark Disk point and the SM951 cost about $.015 per point.
z620: Recently, I bought a low specification z620 (E5-1620 Firepro / 8GB / V5900)and upgraded. This system arrived with three mech'l HD's and the 2011 Seagate 750GB used as the boot drive had a Passmark score of 574. Having had very good results with the SM951 in the z420, I looked into one for the z620. It turned out that the HP Z Turbo Drive used an SM951, but having better PCIe adapter card circuitry and better cooling- a finned Al heatsink, achieves better performance from the SM951.
The z620 receives my "most improved drive performance" as the Passmark score changed from 574 to 13426 - and which equals $.01 /pt. another improvement was demonstrated in the Passmark system rating which began as 2468 and with the CPU and GPU upgrades achieved 2449 and the Disk dropped to 529. After discovering the 8GB RAM modules should be registered instead of unbuffered (the CPU mark changed from 19671 to 22625) and adding the Z Turbo Drive, the system rating on Passmark of 5675- the highest for a z620.
Although there are two results for a z620 with a Samsung 950 Pro NVMe version (results = 12590 and 10085), the other results using M.2 are using AHCI versions.
I should mention here that in my uses and programs, the M.2 speed is so good that all the other components excpet the RAM, including the E5-2690 and E5-1660 v2 are bottlenecks to it's. File transfers are astounding - the GB's fly by- and I would except progrmas tihe high disk /RAM swapping such as video editing will benefit, but starting, opening, and saving seems quite similar. However, what I think of of as the disk core points to cost is extremely favorable and prices have dropped enough that one may as well have the fastest. And, as another poster mentioned, using a PCIe drive frees one of the rare SATAIII connectors- another advantage.
In summary, the Samsung SM951 is an excellent performer at a reasonable cost and given the better circuitry and cooling, consider an HP Z Turbo Drive AHCI version of the appropriate size.
Be certain to update the BIOS to the current 3.90 so the z620 will see the drive as a boot drive. Also, if you use a PCIe boot drive, it can be mounted in only two of PCIe slots which are called 3 and 4.
Cheers,
BambiBoomZ
Analysis / Simulation / CPU Rendering:
HP z620 (2012) (Rev 3) 2X Xeon E5-2690 (8-core @ 2.9 / 3.8GHz) / 64GB DDR3-1600 ECC reg) / Quadro K2200 (4GB) / HP Z Turbo Drive (256GB) + Seagate ES.3 1TB / 800W > Windows 7 Professional 64-bit > HP 2711x (27" 1980 X 1080)
[ Passmark System Rating= 5675 / CPU= 22625 / 2D= 815 / 3D = 3580 / Mem = 2522 / Disk = 12640 ] 9.25.16
[Cinebench R15: OpenGL= 115.78 fps / CPU = 2199 cb / Single core 131 cb / MP Ratio 16.84x