A detailed description of the issue.
Names over a certain length in Steam seem to mess with the game, causing a variety of issues. Some that I've found (username is 2006 Honda Odyssey with XM Radio):
In-game data is displayed after the name - I've seen null, invalid characters, and COLLISION WITH LEVASSUER (or other multiplayer AI)
When fastest laps are achieved, they are displayed as having been performed by another person in the multiplayer lobby (though the long named person still receives credit in points and with the purple colored name)