I've played with people that were moving way too fast to be legit. Am I crazy, or is it possible to hack the xbox version of the game?
Cheating unfortunately does exist even in console versions of games. There are a couple possibilities I can think of:
Artificial lag or "lag switching". A player can manipulate their connection to the servers to exploit the timing window needed to detect a disconnect. While their connection is disrupted but before the server detects it, the player can move to a different location or fire weapons. When they reconnect, the server will simply respond to their new position. Players in the same game will not be able to react to those actions.
A modded console may allow a player to sign in with a hacked version of a game. These are caught and banned by Microsoft when such tampering is detected, but the ban is per-console, not per-gamertag, so the offender can simply mod another console and get back to cheating.