I've never used Sharpoint Portal Server, but stongly suspect that it is similar to Exchange Server in that you can either stream the whole server(mail store) or backup the individual workspaces(Mailboxes), and that when backing up the workspaces, each item in the workspace is backed up individually.
In the Exchange world, we've seen Stores level backup in the 400-500MB/Min range, with mailbox backups of the sme data in the 20-25 MB/Min range.