It's very important that it is put at the lowest setting, otherwise you might end up not having 100% acceleration even when you think you're pressing down fully on the X button.
Look it up on this forum, this sensitivity thing has been discussed time and time again. 10% is the setting everyone should use for the acceleration sensitivity. You're never going to need 50% acceleration, only on or off, so don't put it higher than 10%, it serves no purpose other than slowing you down if you don't press on the button like a psycho.