As long as the upstream O2 sensor is still in place and working correctly, the car will be able to adjust the fuel trim correctly. If you have no emission requirements and you don't mind the Check Engine Light (CEL) being on continuously, you can remove the catalytic converter and the downstream O2 sensor. If you want your CEL to remain off (so that you might notice if something else is wrong), then you can try installing the downstream sensor with spacers. The whole downstream sensor signal can also be spoofed with a pretty simple circuit based on a 555 timer. I believe a diagram for that is hiding in someone's turbo thread IIRC. I'm not sure if a simple resistor or two would work or not.
The above comments are only intended for situations where defeating emission control devices is legal.
-><- Johnathan Valentino -><-
#1 2000 MT, 200k+ mi, owned since 2014, got new old engine in 2016
#2 2000 MT, 200k+ mi, owned since 2015, No A/C, lots of dents, works
Best tank: 69 MPG . . . Recent tank: 64 MPG, June 2016