you can test write, read speed with Aja speed test.
Surprisingly you work with Raid 0, which is a bit harakiri.
If one disk fails you end up with a total data loss.
For video editing the read speed is always higher than the write speed.
If your SAS system is nearly full the speed goes down on a significant level.
The PCIx Slot is not your bottleneck. Even with x8 you should get more data speed out of your SAS system.
It should be around 700 MB per second.
This should answer your question.
Mac pro 8core
several raid systems