In most cases, your bit torrent download speed is directly related to your upload speed. The larger the speed to set to upload, the faster your downloads.
I have a 512/256Kbps ADSL connection, download speed is set to unlimited, upload speed set to 20KB/s. I regularly see download speeds of about 50 to 55KB/s.
Another factor could be using a different range of TCP ports for transfers. Some ISP's now throttle the most commonly used ports by bit torrent clients.