95 Civic has no O2 sensor!

Hello,

My 95 Civic ESi (Philippine domestic version) is a "stripped-down"
version of the U.S. EX model, I think. It has a 'PH16' engine (PGM-FI,
16Valve, 1.6Li, SOHC
non-vtec, ECU code P27) but has no oxygen sensor and no cat converter.
This is common knowledge here, and there is a plug where the sensor
should be. My
question is, is this a great disadvantage in terms of mileage? Average
mileage for civics here is 18 to 24 MPG (~8-10km/liter).

Is the ECU always running in open loop mode? Can we somehow buy an O2
sensor and put it in, and if so, will this buy us much MPG-wise?

Thanks so much in advance.