When we benchmark SATA devices, we consider low queue depths to be the range between one and four. PCIe-based storage scales much higher. Our own Paul Alcorn's enterprise tests routinely scale out to QD256. The SSD 750 can run that high, but it's well outside what any of us would ever encounter. We stop client PCIe workloads at QD128, and even then we're beyond the range of usable performance.
At high queue depths, the Intel SSD 750 really shows its datacenter pedigree. Even as you dip down into more realistic workloads, the SSD 750 operates at nearly identical levels. If I were to guess, I'd say that Intel limited both the 400GB and 1.2TB models through firmware to keep their performance under what the DC P3x00 family can do.
Samsung's SM951 delivers the exact same performance curve between QD1 and QD8. It's remarkable how close these three drives are in this very important metric.