And the iperf test was single stream like mentioned? If yes see how the server performs with internal transfers, best to disable parity first if not that can be the bottleneck.
I believe there have been other reports of issues with those disks when used on a LSI with v6.9, possibly a driver issue, you could try connecting them to the onboard SATA, of course swap with disks from a different model.
This will delete all data on that device like the warning says, there's a way to re-add a device without deleting data, but it's likely to late for that, please post the diagnostics: Tools -> Diagnostics