Button control rate depends on the car you're using I think, it's better to run lower values on high-powered RWDs like the FO8 so you don't end up spinning on corner exit(or locking up the brakes). But you can run higher values on low-power cars like the XFG so you have faster throttle and braking response. As an all around setting I belive I used to have something between 2.00 and 4.00 when I was on mouse, but I was just being really conservative.
I don't know about steer center reduction, I think it's more of a personal preference kind of thing, I used to be at 0.00 or close to it.