Thanks - tkramer, paulj, carbonfiberboy & merlinextrligh for your perspectives. I think this is what I was looking for.

For most of us we either accept what we have (as we did on our first bike) or make a choice to go rim -vs- disc (as we did on our new bike) and learn to live with the compromises.

My guess is that if Shimano or any of the other companies migrating to discs ever did an objective test or calculation to determine the heat capacity of discs -vs- rims and it came out that rims had a higher heat capacity, they would likely bury the results because the company had already decided to migrate to discs (as long as the heat capacity of discs was adequate).