Without, it would detract as currently your eye follows the flow of your car to the stunning rear bumper. I think if you put the spoiler on it you add another dividing line across the car that will spoil the flow and make the car look quite squashed as you already have a difuser eating into the azure paint! This will be especially true if you have it with a carbon underside, if you were to go for it, it would have to be body colourd like the mock up.
Good luck with macking a decision!