Backup-to-Disk performance using a Buffalo TeraStation 5800

[KBE]

Our current backup regime backs up 1.2TB of data to a LTO6 tape drive connected by a 6Gbps SATA HBA.  The data being backed up is pulled from four virtual machines running Microsoft Windows 2008 Server hosted on VMware across a 1Gbps LAN.

We are using Symantec Backup Exec Server version 12.5 with the Remote Agents.

The daily backup job takes 16.5 hours to complete and averages 1200MB per minute throughput. 

We are considering moving to a Backup-to-Disk regime using two Buffalo TeraStation 5800 32TB storage appliance with Western Digital RED hard disk drives which will be configure for RAID 5.  The TS5800s will be connected to the Backup Exec Media Server, (Microsoft Windows XP service pack 3) via a dedicated 1Gbps LAN and the Microsoft Windows XP iSCSI initiator.

My question is can we expect the backup to disk throughput to be the same or better than it is to tape in this scenario?

We have previously tested Backup-to-Disk on a smaller scale using the internal hard disk on the Backup Exec Media Server and external portable USB attached disks and the performance was NOT suitable. 

Does anyone have experience of using a TS5800 or similar for Backup-to-Disk in a similar environment and was the performance satisfactory?