You should note that 600 Kilobits per second (Kbps) (Which your speed test is measured in) is the equivalent of 87.5 Kilobytes per second (KBps) (which your downloads are measured in) and deducting say 20% from that means that you are unlikely to get more that a maximum of 75KBps or so.
8kbps +1KBps
Depending on the hosts you are downloading from you will not always get this speed, but you could download a file from
www.magnetmix.com and see what speed that gives you, which will indicate what sort of maximum speed you can expect.
I hope this helps a bit. It is still possible you may be being capped, but I should check other things first.
Also check you have your speed setting in options set to Cable DSL and the cursor in "downloads" set to a reading of between 75 and 85 KBps