The thanks goes Tob for his willingness to Sticky the thread and any other users who add their results.
Your thoughts are spot-on. Both the elevation and gas did = less horsepower. As I understood the explanation provided by the experts at RWR, the SAE numbers you pointed out are "correction factors" used to "normalize" dyno results to sea level. In other words, the horsepower each car actually put down AT THEIR RESPECTIVE ELEVATIONS was (642 / 1.26) = 509 for mine and (582 / 1.05) = 554 for Softballer's. If both were at sea level, they should put down the "corrected" 642 and 582, respectively.
^^ 100 octane tune? that's it, no other mods?