Where do you live? Is a "silencer" legal in your state? Do you have the $200.00, and the patience, for the federal license?
And also, why? Do you do contract assassinations? Or are you wanting to shoot in your back yard, in a crowded neighborhood?
When you shoot a semi-auto through an effective suppressor, using the appropriate ammunition, the loudest sound will be the trans-sonic "crack" of the passing bullet.
Thus, suppressor users would probably be most comfortable with a load that delivers its bullet at less than 660mph at sea level.
That seems to do away with the Five-seveN, and favor the .45 ACP 1911.