Less Air Plus Less Fuel Equals Less Power.
The UltiMap UM 161 chip programming is the result of many hours on a dyno and represents the optimum fuel and ignition set-up for a PROPERLY TUNED ST2. It will produce more power and better throttle response than your stock Ducati chip at any altitude. Period.
Since your bike runs properly at sea level then you should assume that its basic tune is correct, that is, the throttle position sensor, the throttle body balance and idle adjustment are all correct. This also suggests that your ignition system components are OK and fuel injector nozzles are spraying correctly.
The truth is that EVERY bike will produce less power and poorer throttle response at higher altitudes than at sea level. There's less oxygen in the air so the computer supplies less fuel to compensate. This fuel reduction happens no matter which chip you use.
Here's some background.
Ducati Engine Management System
Ducati’s engine management is provided by an open-loop fuel injection system that uses a computer to control the opening duration of its injectors based upon voltage levels presented to it from several sensors. The longer the injector is opened, the more fuel is supplied to the engine.
The computer contains a Eprom chip that stores a look-up table of injector durations for 16 throttle positions and 16 engine speeds. This chip can be removed from some computers and replaced with one re-programmed with different durations (fuel map) needed for fuel mixture changes necessitated by engine modifications. The fuel map supplied by Ducati is programmed to provide good fuel economy and to meet exhaust emission standards, so does not really provide optimum performance.
The chip also contains a look-up table of degrees of ignition advance (spark map) for the same 16 throttle positions and 16 engine speeds that are used for the fuel map.
The computer determines throttle position from the voltage level sent from the throttle position sensor (TPS) and determines engine speed from pulses sent from a pick-up coil on the engine that indicates crankshaft RPM.
Unlike a closed-loop system that also samples its exhaust gases to make corrections to fueling, the Ducati system only makes corrections (trim factors) to the base map based upon engine coolant temperature, ambient air pressure (altitude) and temperature.
The coolant temperature sensor (thermocouple) monitors the engine coolant temperature so the computer can supply additional fuel while the engine is warming-up and retard ignition timing if the engine gets too hot.
Engines need more fuel when for a cold start as there is no heat in the ports and chambers to keep the fuel atomized as vapor, so it condenses. Fuel as a liquid burns very badly in the combustion chamber, so throwing more at it ensures enough stays as vapor for some sort of combustion. Injected engines get a very nice fuel spray from the injectors and this is why injected engines behave well when cold.
The atmospheric pressure sensor monitors the air pressure so that the computer can supply additional fuel when you ride at lower elevations where the air is more dense. Conversely, if you ride in the mountains where the air is thinner, the voltage output of the sensor drops, which tells the computer to make a pre-programmed reduction to the fuel map injector durations.
The atmospheric temperature sensor voltage level tells the computer how much to modify the fuel map injector durations for a given air temperature. When you take a ride on a hot day, the air is thinner and consequently will cause an engine that has its air/fuel mixture set correctly for a nominal 68°F day, to run too rich. The sensor's resistance increases with air temperature.
Manufacturing and assembly tolerances plus a range of test allowables means that each bike will be somewhat different than the bike that was first used to develop the base fuel/ignition map for that model. So an adjustment method using a Mathesis tool is provided to allow making an additional trim adjustment over a very limited range to compensate for normal production build variances.
On superbikes that use either the Weber-Marelli IAW 1.5M or 1.6 engine control unit (ECU), the trim is made using a potentiometer located next to the EPROM chip socket inside the ECU. The trimmer adds/subtracts a millisecond or so to each fuel pulse over the entire RPM range. On earlier bikes that use the Weber-Marelli IAW P7 and P8 ECUs the adjustment screw is on the outside of the unit.
Your dealer will make trim adjustments with the help of a exhaust CO analyzer. The factory setting results in 1.5% CO for a stock bike to meet emissions but best power is usually realized when full-throttle CO is in the 4-6% range. In any event, check the color of the inside of your tailpipes after a few hundred miles. They should be medium to dark grey, not black and sooty.