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Most OBDII controllers will not let a lean burn happen (unless you happen to live in Australia) the only way you could "fool" the computer into a lean burn is by modifying the pre-cat o2 sensor signals so that the computer would "see" a higher voltage and presume it is slightly rich, causing it to correct fuel trims.
The big device to keep happy is the 3-way catalytic convertor. Even is spark timing is causing a little more NOx than typical, the cat will usually take care of it. But if the mixture moves off of stoich, the cat efficiency drops dramatically and then it can't be your "savior" for a little aggressive spark tuning...
On cold-starts you would not believe how much fuel is thrown down the tailpipe and the timing retard induced just to feed the cat and get it lighting off... I usually fudge those tables a little to improve cold-start fuel economy. As long as the car doesn't idle alot the cat will light off with engine load.
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