I have a similar problem with heat gain on a large hip roof with cement tile. Installation of a powered attic ventilator can reduce the heat gain in the attic but the energy costs of the ventilation outweigh the energy savings of cooling the attic, especially considering the R-30 insulation. Solar powered roof vents have a high initial capital cost but no operating cost, and they provide forced ventilation during periods of peak sun exposure and radiant heat gain.
I have concluded that the payback for forced solar powered ventilation is just too long to justify it, and electric powered fans would actually increase total cooling costs. Adding inexpensive passive roof or turbine vents are the best option I have seen. They won't reduce temperatures as much as forced ventilation, but I figured I could never recoup the capital and operating costs of powered ventilation.
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