Dell SAS5 Performance Issue

I'm suffering from very slow write performance on a Dell PowerEdge 860 with the SAS5/IR controller (mpt driver) running either 6.2-RELEASE or 6.2-STABLE with sources from yesterday. The controller hosts 2 Western Digital 320GB SATA disks in a RAID1 configuration.
Reads approach 65MB/s however writes appear extremely slow, in the region of 6-7MB/s with a dd and a blocksize of 1MB all the way down to about 300KB/s while extracting a ports snapshot.

It was suggested to me that perhaps write caching has been disabled on the controller however no options exist within the BIOS configuration to view/adjust *any* caching options.

Has anyone else experienced this issue? If so how did they resolve it?
Also, is there any way to toggle the caching settings from FreeBSD?

Regards,

Richard Tector
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