Well most BT clients have user-configurable port settings (or just pick a random one) so it's likely that whatever port-forwarding you have set up will not apply to the Blizzard BT client.
Here, grabbed this off their site:
The Blizzard Downloader requires that ports 3724 and 6112 be open. It can also benefit from having ports 6881 through 6999 open.
Hope that helps. I'm sure that the majority of problems users are having with download speeds stem from an improperly configured firewall.