I've played fps games for a while and personally something that I've noticed is that my aim is better when my fingers on the mouse have to do less things, that's probably because if you have to move your fingers on your mouse even slightly then the contact points of your hand on the mouse also slightly change, meaning that the way you aim slightly changes, that's why I like mice that only have a small number of buttons on them, and why I don't bind actions other than fire and zoom to the mouse.