Don't don brick level backups at all?
Seriously. They are extremely inefficient, and CANNOT be used for disaster recovery. And the v5.5 restores were/are a roll of the dice as to whether they would work or not
See http://mail.tekscan.com/nomailboxes.htm for a more detailed discussion.
(20MB/min on a local system sounds about right for speed, BTW, we've seen reports of 5-6 MB/min over a network)