IMO Internal combustion of mixtures leaner than stoichiometric produces products at lower temperature, and with less dissociation of the triatomic molecules CO2 and H2O.
EGT numbers may read higher when you run leaner than stoichiometric, but this is because you are running a incorrect amount of ignition advance. Leaner A/F will always have a slower flame speed so you will have to adjust ignition advance to utilize the power and keep the heat/power in the engine cylinder and not at the exhaust port.