This is something many don't consider too carefully. Let's assume you are sitting dead center and the projector is only a couple of inches above your head. The light striking the center of the screen is reflected back to you fairly directly. As you look at a spot on the screen further from the center the light has to be reflected back to you at an increasing angle. The screen has to have some retro-reflective ability to do this. A matte surface does have a mix of retro-reflective and angular reflective abilities an is therefore the best at producing a uniform image through a wide viewing angle. Anything we do to boost the gain reduces the viewing angle and therefore introduces issues for some viewing setups.Another question:
How is "viewing angle" defined? Eye position to center of screen, or eye position to far edge of screen? Makes a big difference on wide screens, especially if the front row is relatively close to the screen.
You have asked a very interesting question that I don't have a good answer for but I do recognize the issues you are raising. This may be especially troublesome with a Pearl Clear Coat if the flakes are for the most part parallel to the screen surface. It would be very angular reflective and have very little retro-reflective ability.