Are they american by any chance ?, the claims sound american drivel.
A underdriven pulley will offer no benefit whatsoever, the idea that a laternator is a large power consumer is ancient, they are self regulating in that they will only cause enough drag to keep the leckies running and no more, these are normally ecu controlled on yours, so if going slower it just loads the alternator more.
They're just for gimmick people and blingy people, much like the yank flywheels that due to lightness give you + 20bhp, complete crap, they give you zero bhp, just allow faster acceleration(less rotational mass) but bhp remains the same.
tabetha