The drives are very current 2TB Seagate Barracudas, so is there anything else I can do to improve performance? I guess any other solution is going to involve rebuilding the array but I can do that over a weekend.

Peter - I couldn't see (well not in a way that made sense to me) how an extra drive in the array would improve performance, this would be the cheapest option so I'm happy to do that if it's worthwhile.