I have a Sonnet Fusion 4x4 that is half full: two 2TB 970 EVO Plus NVMes and it runs the Blackmagic Disk Speed Test at 6000 MB/s. It was just under.... 5950 MB/s or whatever. Same from read and write.
This is a x16 slot in a 7.1 Mac Pro.
While the numbers are kind of nice, I think we need to look at them soberly. It's not like workflow is based around moving large files around between to Fusion cards... I just did the test to see that everything was working normally: 2500-3500 for a single drive, 6000 for two and 12000 for four. Check. Now back to doing something useful....