Had the same problem but have not actually got around to changing my 02 sensor, its still just unplugged at the moment. Mileage went up by about 80kms per $60 of petrol though. Not sure whether it will get better or worse when I plug it back in with the new one. Before I unplugged it it was running so rich I'm sure there was raw fuel coming out the back end
Lead foot does play a big part though. Drove from Hamilton to Auckland (about 80km) like a Nana then drove a 3rd of the way back (about 25km) at 120km/h & I had used about twice as much gas as I did for the whole distance of the original trip. Went back to driving like a Nana for the remaining 55kms.