Adding 2 switches ain't tearin' it up!!
And particularly the kill switch will gain you more MPG than anything else. When you finally install it, you'll kick yourself for not having done it sooner. It preserves the car's Autostop logic/functionality, so the electric power steering remains active and the engine automatically restarts (quietly, via the IMA motor) simply by: 1) putting the shifter into any gear; 2) depleting brake vacuum; 3) pressing the accelerator. It's all very slick.
Lean burn: monitor the "instant" (bar graph) gauge when cruising at, say, 45 mph. You'll see it spike from roughly ~55-65? mpg up to ~75-100 mpg on a level road in warm weather. That's lean burn. You may also be able to "feel" the transition with a very slight engine stumble. Also, throttle response becomes noticeably "duller".
Prodding the throttle too sharply will cause it to drop out of lean burn. A/C load makes the lean burn envelope smaller too (you can hold lean burn at higher speeds with A/C off).
You can also program a ScanGauge (X-gauge) to simply show it to you, but I went with the factory gauge and seat of the pants mostly. I programmed the gauge to try it out, but by then I had already fine tuned the butt dyno to feel the difference.