The 180 won't affect your mileage at all. The MAF screen can, however. A higher volume of air can require more fuel to keep the ratio correct. It can take some time for the PCM to re-learn, so those two tanks that it was fine might have been while the PCM was slowly adjusting the AF ratio to compensate for the higher flow of air.
That should be minor, however, compared to what an O2 can do. If you're at around 50k on your old O2, that should be your starting point. This is assuming the other 'basic tuneup' items are confirmed. Plugs, wires, etc.