Bosch wideband sensor altitude compensation

I am an experienced tuner and long term 14point7 product user.
This one caught me by surprise when etuning a Mitsi Lancer located near Mexico City.
I got the owner to install a Spartan3 for the tuning process… no problems there but when I went to tune using provided logs to adjust the AFR for maximum power, the results were not as expected. Instead of maximum power occurring near 12.5:1 AFR, it is more like 11.9:1 which would normally result in power dropping.
What this turned out to be is a little known peculiarity of wideband sensors when used at high altitudes. Mexico City is at 2500m which makes it around 75% of sea level atmospheric pressure. Bad news for tuning - you will only get 75% of sea level airflow so only 75% power. But why the 11.9:1 AFR?
Well Bosch has an interesting graph which shows the error vs absolute pressure that explains it all.
The wideband sensor has a significant error as the pressure across the sensor increases or reduces.
So if you have a very restrictive exhaust or live in the mountains, this has to be taken into consideration since no known wideband controller can compensate for this pressure differential.