No, but improving fuel mileage isn't as simple as lowering road rpms, unless most of your driving is at high speed on open roads. The important thing is to operate the engine where it is most efficient - presumably already done by MB when they optimized your car design.
If you DID get lucky and your overall mileage improved a touch, you'd still have to get a heck of a deal on the rear end swap of a perfectly good differential to make the deal financially worthwhile. Plus, you would likely have slightly poorer pickup and passing power. My $0.02.