Changing the port will not improve your download speeds, it is only required in case of a conflict (when multiple programs attempt to use the same port), or if certain ports are blocked by your Internet Service Provider, your company firewall, etc.
If you currently are able to download files without any kind of issue, then you should probably not try changing the port that is used by BitSpirit.
The usual torrent ports are ports that are in the range of 6881 to 6889.