From what I can tell the sensitivity change only seems to effect your turning speed. The up and down motion seems to be the same regardless of where you set it. I would suggest getting used to 2 and working your way up if you feel like you need to turn around faster. For me I like 4 but I find sometimes I need to increase it when the game is laggy to get back to the same feel that I'm used to. I think that I'm running a 6 right now. I also have noticed that if I'm sniping (hardscoping) that I have to increase the sensitivity a couple of levels just to be able to track the enemy players movement because you turn slower when aiming down sight.
Also for getting used to your new sensitivity, go into a private match by yourself and practice spinning and hitting a target like a flower pot or something before going online. Getting used to where to stop will help alot and it's a lot easier to learn that against a flower pot than a guy that's strafing back and forth while shooting you.
i've worked my way up to 9. 10 is too fast. but i can tell you that if i run past an enemy and we have to see who is faster and can shoot faster, it's usually me. it's a huge help, especially seeing how i tend to aim down the sites most of the time (Stalker). i've used my son's profile a time or two... he's on 4. it feels like i'm in quicksand. LOL
I play on 2 to 3 and I've used 5 before. The problem I have with the sensitivity system is the fact that I can't get things exactley where I want them. In the heat of things I jam the sticks and move faster than I need to. If I change it I usually get too slow. It would be nice if the developers would let you select sensitivity for each stick. This would allow me to move faster while keeping my pan/sweep slower.
I have always played with 6/7 sensitivity but if you want play higher than you normaly use, go to private game and just raise it with 1 or 2. Its good training even if you are alone just aim on think, spin and do think like that. That maybe sounds weird but works for me.