I agree the engines develop less power at high altitudes. That's a fact of life with normally aspirated, sea level engines.

I disagree with not adjusting the low needle, and setting up rich of peak. Failure to adjust the low needle sets the stage for "wet" fouling of the spark plug if the engine is run for extended periods of time at mid to low throttle settings. Running rich of peak sacrifices as much or more power as the higher elevation extracts, and again fouls spark plugs. That makes no sense. Cooling air can be maximized to handle the heat generation. A standard cowl opening can not do the job by itself. Unfortunately model engines are limited by their availble ignition and spark plug types and cannot reach their full potential at higher altitudes. Has active in flight engine telemetry been used to verify the engine temps? Without a data plot for the entire flight there is nothing to relate temperature to and actual temperatures over the course of several flights is mandatory in determining what you're engine is doing. A high/low recorder is not a good tool for this kind of information.

Even at 13,000', you're still flying in my "basement".