Well in that case I am assuming two things:
The LSU4 will give a reading of the lambda value - which is the stoichiometric combustion of fuel in air. If the air pressure drops then the mixture will become rich and this will register with the lambda sensor, in closed loop mode you will get the adjustment that you are after.
As as you are using MAP you will get a lower pressure at altitude which means that you won't get to the upper parts of the VE and Lambda table, the Speed Density calculations will compensate for the lower air density (calculated by air temp and pressure).
And most of all, don't rely purely on the sensor, if you have any doubts about the way that your car is running pull and inspect your spark plugs.
