There are two major muscle groups for your chest one is pectoris minor and pectoris minor. By doing incline you don't change the fact that the two muscles work together to lift the weight, the only thing you may change is the leverage scheme, this will stress one more than another.
But the amount of change depends on the change is leverage, so the moral of the story is if you want upper pecs do incline and flat, just to chang the leverage, and be patient......