What you are talking about is called cloaning. It requires you to use something to reprogram your modem (like with a max232) to a modified firmware, which basically allows you to put in another users mac address and get the speeds they are, OR sometimes you can force configs for faster speeds. All in all though, this isn't really the forum you will get much information on, there's plenty of speciality websites that will have a lot more help for you there.
Good luck