First off, mysapce does track ip addresses, but it's not a big deal. The spam limits they monitor for are based on account specific actions, not ip specific actions. You'd have to be doing some pretty heavy moving on their servers before they'd pick up an ip specific problem.
Also, TOR is quite amazing. Above all else... it actually works. A lot of people use it for filesharing servers, spamming, etc. A testiment to the fact that it's fairly effective at what it does can be found by looking at court cases that involve TOR. They're all against people who are running an exit node on the server, not the people who were actually using the node.