The game would easily be a 15. Not just for difficulty, but because of the time required to actually complete every single achievement. This game is by far the longest and hardest 1000 (Extra 250 really isn't that difficult) on the Xbox 360. The only way this game would fit on the 1-10 difficulty chart is if you set up a curve that would change all 10s to 8s or 7s.
Ah, and the servers are shutting down soon, making getting any points on this game impossible. :P