The maximum bitrate 512kbps refers to your bandwidth, it doesn't necessary affects your download speed. For example, if your one download connection is 30kbps, it means you can have roughly 17 connections at 30kbps (subject to operating conditions too) without affecting the average download speed (30kbps of each connection).
That's why subscribing to 3500/6000 kbps doesn't necessarily speed up your downloads although in a lot of cases it does.