Welcome to the ORG.!
For mileage and power gains, I recommend replacing the factory downpipe with a 3" piece. Also, add a high-flow muffler, or straight pipe if you don't mind a little noise. You should also open up the intake with an aftermarket intake or look at the DIY intake setup. You can shim the fuel pressure regulator, increasing the pressure, which will promote better fuel combustion.
The 7.3 is capable of many more miles, if properly maintained.