ok, so i know thats this thread has been inactive for awhile, but i just wanna say my opinion. i use a 4 sensitivity. its great for getting to an enemy, and keeping the reticule on them for the headshots. also, many pros use this. the technique they use to scope in with the br while fighting is helped by this because once shot out of scope, they can still have a good shot at the enemy to shoot them out of scope, and confuse them to get your last shots in. i switched to a 4 sensitivity about 6 months ago, and havent lost a br match yet.
Also, try playing on bumper jumper. it allows you to jumps, beat down, look, and shoot at the same time. 4 is really effective with this because when you jump, you dont have to push the look button to much to make it to their head.
of course, i will also condradict that if you can "ogre twitch" the looks stick, then a 7-8 might be good. the "twitch" allows you to configure a nice "swoop" of the stick so you can get to your opponent quicker. it also helps get use to a higher sens. i havent tried this yet, but it apparently works for ogre 1 and ogre 2, and they recently got a 1mill contract signing deal with MLG...