The AHCI-based SM951 doesn't actually have AHCI in its product name. The new NVMe-capable drive does, though (SM951-NVMe). Keep that in mind when you place your order.
As you can see, the PCIe-based SM951s are up to three times faster than the SATA 6Gb/s-attached 850 Pros in certain categories. We've already covered the Samsung 850 Pro in detail here, and the SM951 AHCI here and here. With that said, let's focus on the SM951-NVMe.
Faced with sequential data, the SM951-NVMe is only a little faster than the AHCI models. The real performance increase comes from reading random data. The SM951 (AHCI) spec sheet claims up to 90,000 random read IOPS. That's less than the 850 Pro, which tops out at up to 100,000 IOPS. Although we managed to pull more random read IOPS from the AHCI drive, it couldn't come close to the SM951-NVMe's claimed 300,000 random read IOPS.
Random write IOPS also improve. The 256 and 512GB SM951-NVMes deliver up to 100,000 IOPS. The smaller 128GB model peaks at 83,000 IOPS. Those numbers are up from the 70,0000 IOPS figure that Samsung ascribes to the SM951 AHCI.
We believe Samsung stopped measuring IOPS performance at queue depth of 32 (SATA's limit). Why do we think this? At higher queue depths, we measured the SM951 AHCI in excess of 160,000 random read IOPS. And using a single worker, we were not able to match Samsung's SM951-NVMe spec of 300,000 IOPS.
Recently, we published data showing the SM951 AHCI we pulled from a Lenovo Ultrabook was slower than the same drive purchased from RamCity. Lenovo has since updated its firmware, though, bringing performance up to the expected levels. You can find that firmware here for SM951 SSDs purchased in Lenovo notebooks.