Maybe one of your drives is jumpered to SATA I compatibility.
Shut down your Drobo and pull each drive one at a time to check the jumpers. You can switch a drive to SATA II without causing any problems.
As I recall, your performance is typical - with a Droboshare. There is a reason the Droboshare was discontinued. If your stated throughput is via Droboshare, try direct connecting the Drobo for a throughput test.
If your stated performance is direct connected then you need to get support from DRI. They can look at your logs to see if one of the drives is not playing well with the Drobo (this happened to me, although my case was more extreme).