Which brings me to another point. I would look at having the fuel pressures checked in case you have a lean condition. Does the engine knock or ping at all under these conditions?
Fuel pressures are all spot on. I have no reason to believe that I'm running lean, but no reason not to either. I don't really know how to check. In any case, no pinging or knocking ever.
As for altitude, there most definitely is an air density effect on cooling capacity. At 1500 m altitude there's about 20% less air for the radiator to transfer heat to, hence cooling capacity is reduced. If the cooling system is oversized then no problem. If not then operating temp will go up. Then add summer heat to the equation, further reducing air density and hence system capacity. And on top of that, [higher altitude]=[lower effective compression]=[lower power]. So you're burning more gas that at sea level to go the same speed and [more gas]=[more heat to dissipate]. It's no wonder that high altitude, high speed summer driving in the Rocky Mtns is very demanding on the cooling system. It's really a perfect storm for cooling system troubles. All the steaming cars pulled over on the side of the road are a reminder. There are LOTS of them.
So, at the end of the day, I'm not surprised that I was running 110C going fast at +2000m in the blazing 38C Arizona heat with A/C on. In my mind that's close to a worst case scenario. Car was running hot, for sure, but never getting hotter and cooled down fast at reduced speeds. That Arizona trip partly as an experiment and a bit scary. I was carrying a lot of extra coolant and other supplies just in case, but if something blew up I was screwed. We're talking about a lifeless, "80 miles to next services," high desert wasteland.
Another variable is the fan clutch, as Rumb suggested. If a new clutch results in improved airflow then temp ought to be a little lower under the same conditions. 105C? 95C? I don't know. But now I'll replace that too on principle. Who knows how old it is.
Thanks, this has been an interesting and helpful discussion!