I don't think you can "hack" your XP. The MP player information is in a binary file. Anyhow, Sjaak, the upgrades and unlocks really aren't necessary to compete. You start off with the most powerful weapons in the game, and you will quickly (within an hour or so) unlock all the attachments for any given gun. Most of the other stuff you unlock is just "bling".
This morning I got votekicked from my favorite server for suspicion of hacking... I am worried that as I continue to get better, this is going to happen more often. Everybody is paranoid about "hackers", when in reality I believe very few people are actually cheating. Skilled players are the targets of "witch hunts" when they start tearing it up on a server.