Reading this thread is a hoot... there is so much misinformation floating around regarding supercharging it isn't funny. The confusion happens because of clowns who try to sell snake oil by calling a $69 12v squirrel cage fan with some flexible dryer ducting an zip ties '12volt supercharger kit'
Something for almost nothing... Hmmmm... too good to be true.
Real honest to God engineered turbocharge and supercharger systems cost thousands of dollars to do correctly. They also take power, lots of it to drive. Anyone ever watch NHRA drag racing, the 8000hp top fuel cars? I've had the good fortune to crew on one of those teams. If I remember right, those superchargers take something like 700hp to turn at the speeds they need to supply 55psi of boost to a 500cu in engine at 7500rpm. If you see a 'street car system' for a 2liter that claims to give even 6psi of boost, but draws less than several hundred amps to drive, be suspicious, very suspicious.
Back to the original posters question/idea: I could see some merit in overall FE if a 'part time' system were employed, on demand supercharging'. When I say overall, I mean using an undersized power plant for the vehicle, say a 1liter in a 2800lb car, 60hp on a good day. Most would never have the patience to drive something like that (most of us on this forum, excluded) but it could get fantastic FE while cruising in that little 1liter's 'sweetspot' . Then use an 'on demand' supercharger to help add 40% more power temporarily, you get up to highway speeds. While in temporary supercharged mode, 8-14 psi, FE would be awful, but you make it up while cruising more efficiently, 90% of the time.
You'd really need to engineer such a system to make it work for FE, would need to increase fuel flow rates and limit ignition timing advance under boost conditions (as to not lean out the engine and melt the ring lands) but this stuff isn't new, an aftermarket programmable ECM would handle this chore. If you chose electric (see link below, an example of a real electric supercharger that supplies power increasing boost).
Controlled Power Technologies... VTES
You'd need several extra 12v batteries that could quickly discharge as much as 350amps into the supercharger motor, than have an alternator of sufficient size to re-charge those extra batteries during cruising. The bigger challenge in a retrofit system would be managing incoming air flow under both conditions, blown, and high efficiency normally aspirated. In the electric supercharger in the link above, it looks like they slow the rpms down during non-boosted operation, drawing only 1.5amps. Mercedes has/had a part time belt driven supercharger system, that manages this airflow problem somehow. Some type of flapper door that toggles between one incoming air route to the other?