RN3220 / RN4200 Crippling iSCSI Write Performance

I've now swapped into the array an Intel DP NIC and have linked the 10GbE into the system and it's offering even worse performance.

Intel DP ET 1000BASE-T Server Adapter

Switched

9k frames

Tested as one port and dual port RR MPIO

Slightly worse performance in read and write than the built-in adapters

QLogic BCM57810 10GbE 10GSFP+Cu

Point to point on a 3m SFP+DA, no switching involved

No Cluster Shared Volume

On a dedicated 1TB LUN

RAID0 underlying array

Nothing else happening on the array

All other optimisations as above.

So I think it's safe to assume it's not any of the NICs involved here.

First few lines of TOP during a benchmark
%Cpu(s):  0.0 us,  1.9 sy,  0.0 ni, 97.7 id,  0.3 wa,  0.0 hi,  0.1 si,  0.0 st
KiB Mem:   3926940 total,  3679804 used,   247136 free,     2196 buffers
KiB Swap:  3139580 total,        0 used,  3139580 free.  3318992 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
 6306 root      20   0       0      0      0 S   4.0  0.0   5:23.73 iscsi_trx

Internally, do a lage file copy within a VM and it'll settle at 19.8MB/s

So basically I've inherited several 4x1GbE disk arrays that on 12x 7200 rpm Enterprise SATA drives in RAID 0 cannot write at more than 690Mbps in occasional, yet quite infrequent bursts?