Expecting faster backup rates from iSCSI san into Backup exec 2012

Hi everyone, 

I have a Dell MD3000i iSCSI SAN (14 600GB 7200K SAS drives in RAID 5)

directly connected (1 Gig ethernet) to a Dell Poweredge R710 server with Backup exec 2012 installed to it.

I also have a Dell TL2000 tape library with an LTO5 drive connected to the Poweredge R710 using SAS using Symantes supplied drivers..

The LTO5 drive is configured with a 64KB Block size, 1MB Buffer size and everything else set to default.

A full backup of my data (around 8TB) takes approx 4 days on the TL2000, with an average data rate of 950.00 MB/min reported in backup exec.

I was expecting higher transfer speeds and am having trouble determining the speed bottleneck.  Can anyone help determine why it's taking so long to backup 8TB data onto LTO5?