I live in Albuquerque at an elevation of about 6000 ft. The temperatures you talk about are about normal for my 93 300E2.8. I have a 50/50 mixture of MB antifreeze, nothing else, but the cooling systems is super clean with no gunk in it. My temps might be 5 degrees cooler but how well are the temp gauges calibrated anyway? I don't turn off the air conditioner or do anything else to try to get the temperature down. On 90 degree days with the a/c going, I always thought it was interesting when I climb the big hill for the last mile home to my house that the temperature actually starts going down as I reach my driveway. I guess the thermostat is wide open and all the fans are going full tilt. I have had the car since new and there are no temp changes with age that I can tell. I have a couple of other newer MBs and they act about the same.
I drove to Santa Fe last Friday. Outside temperature was about 95 and I had four people in the car. At 75 to 80 mph, I noticed it actually ran a little cooler even on the upgrades than around town.
As far as the power loss, 95% of my miles have been above 5000 ft so I guess I do not notice or have nothing to compare with.