According to Recardo's studies, peak thermal efficiencies generally occurred at about 15% lean which would put the AFR at roughly 16.9, that is to my knowledge, a typical number and is due solely to the fact that combustion temps are colder. Any leaner and flame speed drops off drastically.
According to Stone, highest flame temps occur just rich of stoichiometry, however, sufficient oxygen must be present for NOx to form so it occurs just lean of stoich.
Low ratios of EGR dilution significantly reduce NOx emissions. The catalyst is the booger, it must receive a burst of rich followed by oxygen; a continuous cycle.
Credits to Toyota Motor Sales