Center the tanks and the head pressure is +2.5 inches at the start and -2.5 at the end, Total change = 5.0 inches. No matter where you put the tank the total change will always be the same. Centering the tanks does make life easier in other ways, but it does not cure the leaning out during flight.

Doing the math - 1 inch of alcohol creates 0.02926 psi, so with a 5" tall tank the pressure at the bottom of a full tank is 0.1463 psi. If you have 1 psi pressurization from the pipe that means that the fuel delivery pressure to the carb is going to reduce by about 15% from the start to the end of a run. That will create a noticeable change in the mixture.

Steven