I am having the exact same problem.
We've got a HP NAS 2000s, which is basically a DL380G3 with a SATA disk array hanging off it. It runs Windows Server 2003 Appliance Edition.
We've got a Quantum SuperLoader with a HP Ultrium2 drive.
I just rebuilt the NAS the other day due to severe stability and other issues. Installed BE10, installed SP4, so i'm currently at 10.0 Rev5484
We noticed that the backups were going very slow (350-450MB/Min). At least half the speed of their pre-rebuild state.
I then checked the drive properties, and noticed that there is no LTO-2 media listed in the allowed media types. BE seems to pick up the stats that the drive is an Ultrium2, but the media type listed is simply 'LTO' in the 'General' tab of the drive properties. From memory I thought it was supposed to say LTO2 there as well.
I then checked the media that I had in there, and the tapes were not being recognised as LTO2 tapes.
Oh, and i'm using only Veritas drivers. I haven't installed anything prior to installing BE.