Well, actually after your last comment I went there to have a look and I discovered that as long as I use
Input.GetAxis("Horizontal") I can be assured that it is frame-independent by default so I shouldn't be worried about Time.deltaTime in this case, I can leave it out. I guess the only option here is to have an option menu where the player could set the sensitivity, that should solve the problem, yes a slider in a settings menu would be good.