Slow data transfer speed with Ultrium 1840 in VMware Guest OS on ESXi 4.1 host

[JohnQuest]

We have a Dell R710 server with an Adaptec 29320LPE SCSI adapter card installed. The OS is ESXi 4.1. The ESXi host sees the card just fine.  Attached to that is one device, a Ultrium 1840 External SCSI tape drive. Drive is seen and can be added to the Guest VM which is a Windows Server 2008 R2. This is all done via VMware KB 1016407. Using HP StorageWorks library and tape tools 4.13 SR1, the tape drive checks out fine with no errors or warning. We use Symantec Backup Exec 2010 R3 and jobs can use the tape drive just fine.

So everything is hunky dory OK, right? No! The negotiated SCSI data transfer speed is only 80 Mbps. That is slow, jobs take longer.

When the drive was previously connected to a physical server, the SCSI data transfer speed was 320. If I reconfigure the ESXi host to passthrough the Adaptec card, the Guest OS then sees the card and the tape drive and the negotiated speed is 320, so everything is working.

Is there any way to get the Negotiated SCSI data transfer speed higher when it is configured the recommended way?