Also reading some of the threads I can provide some insight, I used to work as a paint chemist for BASF who supplied GM at the time. White BC is high high solids full of TiO2, very hard to apray smooth in a production invironment. The way we deal with it at where I work is several light coats then a lot of clear.
I have always heard the rumor that GM specs some OP to hide dirt and some customers mistakenly think it means more paint=better. Where I work we shoot for the least orange peel possible.
Also it is absolutely true that the ciba UV protectants in the clear are the most expensive portion of any clear, ironically it is also the lowest % by weight in any formulation. This is why you never buy cheap after market clear they cut back there first. Lower mils does equal less sun protection. Test are run at 1.0 mils in florida exposure at most OEMs, some want 1.5 mils, one company 3 mils. Some want no yellowing or oxidation in 5 years, others 7 and so on. You never know. But in every case same clear more thickness = better protection.
And don't think every car makes it out of the assembly plant with a 1 mil of clear over the whole vehicle, mistakes happen alot. You may be lucky and have 3 mils on the hood, or the operator may have been asleep when your car went by and missed two passes leaving 1 mil, inspectors never check the roof.
Thats why I always keep my car out of the summer sun and am considering wet-sanding and clearcoating. But then again its just a Yaris (that I want to drive for 20 years.)