Exhaust Leak

[DesertRace21]

I know it has probably been talked about on here before, but I have to ask some questions. I have an exhaust leak. I am pretty sure it is just a warped manifold, but rather then pulling them off and sending them to get re-surfaced and re-installing them, id like to add headers for whatever benefit they are worth. I know my gas mileage should see a hair of improvment, but in theory, such as on a nissan, with the exhaust pulse and a warped manifold, the O2 sensor shold be getting a false reading of straight fresh air and telling the fuel managment to fatten that bad boy up right? I have a 2000 Z-71 with a 5.3, but it is lifted and on big tires, however i dont think i should be only getting like 12 miles to the gallon. A stock 5.3 was doin way better.. and i think a lot of my problem has to do with the d**n manifold being warped.