The skill ceiling is dramatically higher with a mouse and keyboard, particularly when the game allows it, i.e. fast-paced games with dodge mechanics like Unreal Tournament.
If the game is a console shooter to begin with, it's going to be so slow that the benefits of a mouse will be much less. But even then, the skill ceiling is noticeably higher. --- Console war in a nutshell: http://imgur.com/xA6GJZ9.png