Unfortunately, the iMac 5k includes an older 2-lane PCIe Sandisk SSD, at least in the 256GB configuration, which means SSD performance is more comparable to a SATA SSD than a PCIe SSD.
Ideally, the iMac 5K would include a 4-lane PCIe M.2 PCIe slot, as found on the latest PC motherboards, so we could upgrade to a newer, faster SSD.
For the non-techies, the storage hierarchy is as follows:
(1) M.2 PCIe 3.0 X4 (up to 32Gbps theoretical or 3.2GBps real world max)
(2) M.2 PCIe 2.0 X2 (up to 10Gbps theoretical or 1.0GBps real world max)
(2) SATA Express (up to 10Gbps theoretical or 1.0GBps real world max)
(3) SATA 3 (up to 6Gbps theoretical or 600MBps real world max)
(3) M.2 SATA 3 up to 6Gbps theoretical or 600MBps real world max)
(4) M.2 PCIe 2.0 X1 (up to 5Gbps theoretical or 500-550MBps real world max)
Again, the Sandisk 256GB SSD in the iMac 5K is an older M.2 PCIe 2.0 X2 (X2 = 2 lane) device. The theoretical max for that interface is 1Gbps, although the Sandisk drive uses older technology so its actual throughput is 600-700Mbps.
You must have both a motherboard and a SSD that supports the same interface to achieve a given level of performance. If you stick a M.2 PCIe 2.0 X1 SSD into a M.2 PCIe 3.0 X3 slot, then you will get a max of 500-550MBps, not 3.2GBps. Similarly, if your motherboard only has a M.2 PCIe 2.0 X2 slot, then you will never achieve more than ~1.0GBps no matter what drive you use.
Samsung just released ithe successor to the XP941 PCIe SSD found in the newest MacBooks. This device, the SM951, is a M.2 PCIe 3.0 X4 SSD with NVMe that delivers 1600MBps throughput (assuming a motherboard with a a M.2 PCIe 3.0 X4 slot) with much lower latency, or nearly 2.5X the real world performance of the 256GB SSD in the iMac. I'm hoping Apple opted for this drive in the 1TB SSD configuration; if they did, it would be a worthwhile upgrade over the 256GB model.
if the 1.0 TB PCIe SSD in the 5K iMac (have one on order) will be connected using only 2 lanes, I think I will return it. this would be inacceptable. I now have a 4-lane XP941 running in my classic Mac Pro and I do not want inferior performance.