WHAT DID WE LEARN?
First off, note the difference in WRITE speed between the 970 Pro and 960 Pro NVMe flash blades. For some reason, the 960 pro has always had write speed issues with Thunderbolt 3. Thankfully, the 970 Pro overcomes that.
You may wonder why a four flash blades capable of 3500MB/s READ speed each can only achieve 2500MB/s when stripped. Easy answer: Thunderbolt 3 only has so much bandwidth and tops out at around 2600MB/s. If you need more speed, you could get TWO flash storage units and bridge them across multiple Thunderbolt 3 controllers, thereby achieving over 4000+ MB/s. (The 2017 iMac Pro, the 2018 Mac mini, and the 2017/2018 MacBook Pro have four TB3 ports, each pair on separate controllers.)
Of course, the whole point of the Express 4M2 is increased capacity. With four 2TB flash blades (in the four M.2 NVMe SSD slots), you achieve a total of 8TB capacity by striping them (RAID 0) as we did using SoftRAID. (NOTE: We verified that the 4M2 also supports AHCI blades like the Samsung SM951.)
As you can see by the image below, the Express 4M2 includes a pass-through Thunderbolt 3 port and a DisplayPort.