I play on 3. I've started to play cod like I do CS now that I know the maps and its very effective. What I mean is I just have proper crosshair/ads placement so I don't need to adjust much or even at all.
? Thats slow man. I don't know how you'll keep up with any form of stalker/lightweight/strafing lmg players with a 3.
Right now I'm getting used to 7. If that doesn't work out I'll go back to 6.
Sensitivity in this game seems more important than other games. With some crazy spawns on some maps as well as lack of footsteps, it seems more beneficial than other games to play on a high sensitivity. 7 Isn't exactly high, but it's higher than what I used to play on.