Dude ecu works on inputs of airflow and other various sensors, which you already know. So don't give this silly judgement that it will run lean.
It's not running lean at all.
The stock 1.3 N16 does not have o2 sensor.
So basically by upgrading to O2 sensor and ecu will make the ecu decide on the basis of o2 sensor input and it will enhance the response, economy.
Else I've tested QG15 /13/18 all ecu's interchangeably on different setup.
So basically if you install a QG18de ecu on qg13 nissan sunny, car won't say sorry i will not start. And it will also not say hurraaah! Ill throw more fuel down the exhaust.
So I've checked this car by using wideband afr gauge aem to see what's the afr and its running in 14.5-16 range.