It really depends on the game on Halo I play on 5 just because i need nice steady aim when I strafe. On COD different story I Play on 10 sens just because its a fast pace environment
mine is on 8. with these turtle beaches as soon as i hear something like a gun reloading i can be turned around and shooting in a blink of an eye great for hardcore. but it did take some time to get use too.
I think sticking around 5 in COD is the best because its quick enough to turn on someone but not so fast that you struggle with accuracy when you get in a gun fight.
Been playing on 4 for a very long time, too late for me to change. In the past I have tried out higher and lower, but nothing keeps me on target like this one does.