Is this black ops? when you are in your secondary sensitivity screen, make sure the "game" field says "Black Ops ADS". that translator will make ADS faster. Just know that the speed scale is different when aiming, and the sensitivity on the ADS translator will most likely be slower than your standard config.
on xbox games, Aiming is always slower than regular looking or "hipfire". You won't be able to do a 180 degree turn while aiming on xbox. you should be able to do at least a decent 90 degree turn though. But you'll have to learn to drop your sights to make quick turns and then raise your weapon again.
it shouldn't be that bad though.. you still should have decent movement control while aiming, just not super fast.