Poor performance backing up to iSCSI target

One of my friends is facing this situation. Please help. Here are my details:

Media Server Type:  Virtual (ESX 4.1)

Operating System:  Windows Server 2008 R2 Standard - fully updated

iSCSI Appliance:  Dell MD-3000i

NIC Configuration:  (2) VMXNet 3 - one on the LAN and one on the iSCSI (isolated) subnet

Backup Exec:  2010 R3 fully updated (Version 13.0 Rev. 5204 64bit)

Note:  The Dell MPIO driver is installed and iSCSI is configured per Dell's recomendations for Server 2008 R2

Exchange to iSCSI B2D Speed:  550 MB/min

Exchange to C: B2D Speed:  1350 MB/min

Media Server to iSCSI B2D Speed:  1150 MB/min

Windows Explorer C: to iSCSI Speed:  3000 MB/min

In other words, if I back-up the Exchange server to the Backup-to-Disk folder on the iSCSI appliance I get less than half the throughput I get backing up to the C: Drive on the Media Server or backing-up the Media Server to iSCSI.  If I do a straight file copy using Windows Explorer I get about 3000 MB/min.  I'm not particularly impressed with 1 GB/min but I can work with that.  500 MB/min is painful for the size of our Exchange environment. 

Any ideas out there?