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I wouldnt go changing the o2 voltage by force. Narrow band stock o2s are only accurate at .5 volts(some sources say .45) and that is because they act like on off switches. Higher than .5 means rich, lower means lean. When you change the voltage from .5 to .6 it just switches at .6 causing the car to constantly think it is running rich and constantly build negative fuel trims.
O2 sensors are used in a closed loop feedback system to generate these fuel trims. Most cars have short term trims which run directly off the sensor reading in real time, and long term trims which are based on historical data that your ecu stores and are pretty much an average of your short term trims. You can log these fuel trims using any obd2 logger and you will always get the best economy/performance/drivability when they all average out to zero for every load/rpm. This can only be achieved with some piggyback tuning as there are variances manufactured into the car.
The only way I would suggest altering your cars idea of Stoich or the target closed loop afr is to get an Innovate LC1 wideband controller which has 2 programmable analog outputs. You can set one to read .1 volt for anything leaner than say 15.5 and .9 volts for anything richer than say 14.9 this way your computer will see a switching voltage of .5 and you can accurately tune what the appropriate switching afr is. Also, you can mount a cheap 25$ afr guage and program the LC1s secondary output to be say 1 volt at 12:1 and 0 volts at say 16:1 and get an idea of where your car really runs and also monitor your open loop operation to make sure it doesnt go too lean.
Your ecu bases its open loop operation on the closed loop developed fuel trims. It is basically guessing what ammount of fuel will be necessary to achieve a safe afr under heavy loads. If you are at wide open throttle(WOT at 2000 rpm is like open 30% before someone says you will never save fuel at WOT, you also would never make it up the hill at 2000rpm without going to WOT) going up a steep hill your engine wont last very long running a 15:1 afr. Be careful how lean you run in closed loop because it will have and effect on your open loop operation.
BTW... Lean burn engines tend to have staggered intake cam profiles within the cylinder so if you have 4 valves per cylinder you might be able to get custom ground lean burn cams. Wiki search the MVV ( Mitsubishi Verticle Vortex ) engine for more info. BTW most cars do have a slightly staggered intake cam profile anyway so we still might get to burn leaner efficiently without a set of cams.
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