I'm not qualified to discuss Windows Storage server...but I can talk about latencies.
10-15ms for SATA disks actually isn't that bad, and doesn't indicate a major problem. That *is* on the edge of starting to become an issue (for example, Oracle wont talk to you about performance issues if you get above 15ms)...So I wouldn't expect you could put much more workload on those disks, but they aren't doing poorly right now.
*generally*, 10-15ms is considered the upper bound for 'good' performance.