Optimum combustion is rather subjective. Optimum combustion could mean maximum power, maximum mpg, minimum emissions, all of which will be different depending on the engine in question.
Stochiometric combustion, (theoretical combustion), is considered to be the ideal combustion process where fuel is burned completely, (simply a balanced chemical equation).
Since catalytic converters are designed to function optimally at 'stoich', both statements can infact be considered true.
And yes, piggyback ecu's are very much about compromise, especially with the K11 ecus.
