How can we presume that the supplied Intel cooling solution is a limiting factor? It cools down to 67C with AS5, but only to 77C with another paste. So it seems that it's not limited at all based on output.

If you use an aftermarket cooler, you lose the ability to have a wider delta. You also lose the 1" contact area which will remain consistant in every test, and you'll lose the level mounting pressure.