High fuel consumption = Bad 02 sensor?

I am currently driving a derivative of the fsde which is the FSZE 2.0. Admittedly, I have a heavy foot but I have seen that my fuel consumption is getting a bit ridiculous, think 11 mpg city and 20 mpg highway.

It has an aftermarket standalone ECU which was tuned to make a little more HP. There are no engine codes.

I am trying to think what I can replace to improve the mpg. I have cleaned both the TB & MAF sensor which weren't actually too dirty. Replaced the IAC. Plugs replaced. New Air filter. Brakes aren't stuck as I have just had them changed. I burnt through a bottle of techron fuel system cleaner also. It doesn't smoke when starting cold or at WOT. I redline it quite regularly.

The FSZE has a single upstream 02 sensor. Would you recommend this being changed? Any other solutions? Maybe a whole new exhaust system? Would new injectors help? I am based in a country where labour is extremely cheap, like $7/hour is premium for a mech. My last resort would be to lpg swap it for $1000 which would pay for itself after a year.

Thanks for sharing your thoughts.
 
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