My attic is 1.5 inches of foam above a sloping concrete deck, there is white metal roofing. Maybe R7 total.
As far as keeping the heat out I think it outperforms R30 at the ceiling plane in a vented attic.
Before I turned on the AC the attic was a degree hotter than ambient. I believe that was better than a 140F sauna with a fibreglass air filter in between the ceiling joists.