May I ask your opinion on this info?
http://duro-last.com/solar_reflectance
They claim an SRI of 108 for their PVC compared to other materials on the chart below. I wonder how it stacks up to the TPO you use, and other similar products? Here's an excerpt:
Roofing System / SRI / Reflectivity / Emittance
White EPDM 84 69% 87%
Kynar Coated Metal, White 82 67% 85%
New, Bare Galvanized Steel 46 61% 4%
Light Gravel on BUR 37 34% 90%
White Granular Bitumen 28 26% 92%
Dark Gravel on BUR 9 12% 90%
Black EPDM -1 6% 86%
[edit] just found this link with some answers from Firestone/GAF
http://firestonebpco.com/assets/2015/06/tpo-white-paper.pdf