[aerohead]

The reasoning according to ASHRAE is that, if you don't have living space on the inside of the 'roof', or AC equipment and ducting in the attic space, you're throwing money away.
Thermal insulation works on the difference in outdoor and indoor dry-bulb temperature. It doesn't experience' infrared. However, check your temp where the roof pitch is near the attic insulation. If it roof infrared is 'baking' the top of the insulation, it's going to raise the air temp on top of the fiberglass batting or blown-in fibers, cellulose, etc..
If exterior walls are exposed to solar incidence, 'Tech-Shield', or something like it ( OSB with foil facing ) could help.
The best thing is just to keep the sun off exterior surfaces ( load avoidance ).
Trees and landscaping can do that. Solar fabrics and awnings.