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8.9 is what the manufacturer states that a particular HD can peak at. I personally dont think 15 is terrible but I guess that depends on your hardware. If it were me I would try running a few more test just to see if that number fluctuates.
I benched a WD 80GB/8MB cache 7200RPM drive with Sandra and my ave. seek time is around 12ms. And I don't have much info on the drive or much mess so I would think that 15ms is ok.