It is a nice idea, i imagine you had a lot of ideas of things to implement but couldn't because of the time.
One thing i thought about was starting with the silencer and having the enemies that are closer hear the shot, that way the game would be about position yourself in a way that you could handle the amout of enemies that each shot would alarm.
But then, if you start with the silencer there wouldn't be an upgrade xD.
Good Job.