We talked about intake leaks, but what about exhaust leaks? Can an exhaust leak affect performance? Yes it can, especially if it's a leak before the O2 or AF sensor. If there is a leak in this area, it will give a false reading to the O2 or AF sensor, thus upsetting the fuel mixture. So if you have poor fuel mileage and an exhaust leak, you might want to address the exhaust leak and recheck your fuel mileage after the repair. The improper fuel mixture created by the leak can also cause performance problems, so don't rule out your exhaust leak causing a problem.
More information on how to find exhaust leaks can be found in the Noises article.
Lastly, if you have O2 sensor codes indicating a lean mixture, don't replace the O2 sensor. Look for exhaust or intake vacuum leaks first, repair those, and recheck for your codes.