Having tested the described IOMeter test on 12 different machines using a variety of hardware I have narrowed it down to this specific chipset (3 different machines with the same chipset exhibit this behaviour using RAID1 and RAID10).
I have also managed to rule out the driver as I have a machine with a different chipset (that doesn't exhibit the behaviour) that uses the same Intel Matrix Storage driver version.
I have tried Intel Matrix versions 7.8 and 8.8 and they both exhibit the same problem on the affected machines.
Can Intel confirm this is a hardware issue?
Has anyone else seen this?
Can anything be done about it other than buy new RAID controller cards for the affected machines?