You have to remember, necessary is relative. You live in a place that does have a lot of sunshine. I have not experienced a summer in my Y yet, but have in my wife's model 3. The roof glass can get warm, but depending on your tolerance level it was not that bad to me. It never really bothered my wife either. Now saying all of that, I did tint everything but my front windshield, but it was more for the aesthetics, so not really "needed".Do you think its necessary to add another layer of tint? Or does it feel super hot?
The factory "tint" however is just that, colored glass, so the same as non-colored glass if you are concerned about heat rejection, the factory tint isn't it. Also remember, all tinting doesn't reject heat, so if you are focused on keeping the car cool, you have to make sure the tint you are getting performs well.