Even if your alternator wasted 400 watts of energy, that's only about 1/2 Horse power. The gains in MPG we see is upwards of 10%. If our autos used 25HP to drive on the Hwy, then 1/2 HP would only be 2% gain.
I dont believe no load alternator waste anywhere near 400 watts of energy. Big Rigs use way more than 25HP and still see 10% gains. Now through in all the efficiency losses of the alternator and cell. So recouping wasted alternator power (more like 1/20th of a HP IMO) could not explain the gains in MPG.
(Maybe I'm over simplifying a little but Philldpapill did a good more comprehensive so here's my version)