I've looked into this as well. The best claim I've ever heard, including from a manufacturer, is that you don't lose mileage (which I have a hard time even believing that, since it takes some power even to spin a naked assembly).
The only real way to truly gain MPG with a supercharger is in conjunction with a smaller motor. For instance, swap your NA 4.3 for a supercharged 2.8.
BTW my own prediction is that in the next 10 - 20 years, if we are still using gasoline engines, it will be with very small (0.7 liter) motors with huge turbochargers (30 - 40 #s of boost).
-Bob C.
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