Let me throw in some food for thought... my brother spent about $8 grand having his attic completely reworked. They came in, sucked out all the insulation, sealed off all gable, soffit and ridge vents, then blew on the expanding foam on all exterior surfaces including the gables and the underside of the roof.
We live in South Carolina and our summer days are typically in the mid 90's. If you go into a typical attic in the middle of the day here, it will be in the 130+ degree range.
He told me the temperature in his attic never varies more (or less during winter) than 7 or 8 degrees from his living space. He is very happy with the outcome and his energy bills are down considerably. So, with that, I recommend spraying the underside of the roof.