Those who have aftermarket, stand-alone engine management computers have done some interesting things with lean operation on very modern engines.
Lean operation requires more throttle opening, and the throttle by wire can compensate without driver input. Driveability issues are minimized with such setups.
My current "fun" car is a 2004 Turbo S2000 with stand-alone ECU, 8.8/1 compression, 19.5Lbs boost. It does not run lean under light loads, and does not return very good MPG's. (about 20) . My plan is to re-map the ECU for 17/1 operation under light loads. Those that have done this see overall increases to 25 to 27MPG.
It's my understanding that modern direct injection engines have been configured to operate smoothly at much leaner A/F ratio's. And coupled with high compression, can achieve BSFC numbers almost 25% better than the very same engine operated at stoic.
Take a look at the lower picture. That's my 1971 Cessna 177RG. I'm operating it in very lean mode! (GAMI injectors and other tweaks) I'm achieving a solid 25MPG at 125MPH. I promise, there is no tailwind and I'm actually climbing in this pic. What I do is lower the RPM to 1900-2000, pull throttle to 17 inches manifold pressure and lean it to it loses significant power and misfires a touch, then richen it just a touch. I then fiddle with the controls to achieve a bit less than 5 gallons per hour fuel flow. (the gauge is just to the right of my iPad)
Note: in non lean operation, the same speeds consume about 8GPH.
Look at the airspeed indicator, 113MPH at 4500 feet. That's 125 true airspeed. (remember, the higher you go, the lower the airspeed indicator is for a given speed, due to thin air)