Leaning fuel mixture and effect on engine temp

[Flatland2D]

I've heard that running a leaner fuel mix will make the engine run hotter because there isn't extra fuel to absorb some of the energy. I am considering making a microprocessor based EFIE-like device that would hijack the O2 sensor signal and scale the signal higher or lower so you can fine tune your mix. I'd also like to add a bar graph of fuel mix for aid in tuning. I have it all designed, it's just a matter of buying some extra parts and piecing it together. My only reservation is this possible rise in engine temps. Should I just keep a close eye on the thermostat, or should I wait till I can rig up a water injection system?

If all goes well with my device I will probably post plans. I don't know if it's really worth the time and effort to get into selling fuel saving devices.