First, max out Block Size, Buffer Size, and Buffer Count.
Then take a look at this:
http://ftp.support.veritas.com/pub/support/products/Backup_Exec_for_WindowsNT/281670.pdf
Page 12, the note under the VXA drives.
"1. Capacity and compression reporting is not currently supported for this device."
So if you are looking at the stats that BE is giving you, they are bogus.
If it is spanning tapes on you, then you have a problem...