Hi,
If the RAID Controller is RAID 10 capable, stick another drive in the box and run RAID 10. RAID5 spends quite a lot of time calculation the data offsets whereas RAID 10 doesn't.
On a highly loaded system it makes quite a lot difference performance wise.

>>Hint: for such tasks it usually pays off to use different disks with input and output files if you have control.
>
>It's a relatively new machine with an array of SCSI drives running RAID5. I doubt that they're very fragmented. And you just can't beat that kind of disk setup.