So, after we paused at the shady alcove, watched another rafting group (a private one this time) go through Lava Falls Rapids both upside down and without rafts in some cases), we continued down to our campsite with Kris and Andy in the duckies. Our campsite at Whitmore Wash, mile 188, was in full sun, even when we arrived after 4.
After arriving and unloading the boats, I convinced Kris and Andy to try Kim's trick of watering down the tarp area to cool off the site. They were both skeptical of the idea, but willing to try.
Michael commented that the thermarests provided enough insulation from the ground to make the ground cooling beneath us unneeded. I countered with the argument that the ambient heat rises from around the thermarests.
We all agreed that the proper answer was, "We'll see."