It would be interesting to see an actual comparison of the two. I have flat white as well as silver bubble wrap and a removable Mylar panel. The Mylar panel is, it seems, the most reflective.
The comparison was nowhere close, as I recall. White paint is over rated for reflection. Light can pass right through paint ya know. Then it's just wood after that, or whatever's behind the paint. Like if you put paint on glass it doesn't stop it much at all. Maybe a bunch of layers but not 1 or 2. Same with mylar actually, but not as bad.
You know those mylar windshield screens to keep the car from heating up? Well I put one in front of a chamber where light came out at the bottom and I can see light blasting through the mylar and the foam. It ain't like foil, where no light gets through. Maybe foil and then mylar on top the foil would work well. BTW, if you ever wanna look at a COB or other LED while it's on, two layers of emergency blanket makes a nice thing to look at them through and not go blind.