Comment 22 for bug 1668557

Only diff is max_sectors_kb, 512->4096

That matches the iostat, 8k there is in 512b sectors which matched 4k max sectors.
I don't see where the patch is that breaks that in linux-stable, but it is worth a try to boot a bad kernel and set 512 in /sys.

Later to tune for your raid case with so many disks you might consider setting also
/sys/class/block/sdb/queue/rotational = 0
Locality doesn't mean a lot to your setup and might safe some cpu cycles.