IO540
You are correct about saving 20% on the fuel burn by leaning to peak however you are still stuck with the ignition timing fixed firmly in one place.
The LASOR ignition system uses inputs from RPM and MP to move the ignition timing to the optimum position for the power setting and saves fuel in all phases of flight.
A FADEC will take this one stage further by enableing the engine to run so lean that it is right on the cusp of detonation, Lycoming are saying that with the FADEC they are working on the engine will in fact be four engines (or in your case six) conected to one crankshaft as each cylinder will be individualy managed.
In short the FADEC will be able to achive the sort of mixture control for each power stroke that you or I can achive only after we have been in the cruise for some time and it can also adjust the ignition timing, that we can't do.
Admitting this box of wiggly amps is better at controlling the engine comes hard from someone who's ultimate job would have been flight engineer on a Lockheed Constellation but that's what they call progress!