Have you looked into SAN SSO with Backup Exec?
It will allow the full speed of the SAN to be used, and will cut down on backup times too. You would need a full installation of BE on each server that you'd want to backup across the SAN (so ideally your file and print server, Exchange etc...anything with a large backup size).
Using the method you're using, I suspect it's running your backups across your LAN, which means you need to look there...it would have nothing to do with your SAN in this case. Here is a link to follow:
http://support.veritas.com/docs/258734 <-- BE Admin's Guide