47% increase in gas mileage!

Well that means before the sensor fix you were running rich. The down side to this is excessive fuel can damage a catalytic converter. They are sized to the engine and if more fuel than they are designed to operate in, they "run off" and melt the catalyst clogging the exhaust. Hopefully this wasn't the case so if you see degraded performance and sluggishness check the converters. was it the pre or post cat o2? Did you reset the ecu?