[mpg_numbers_guy]

Yes, it mostly does just produce extra losses at higher RPM, but more power is available at lower RPM than otherwise, and for max regen braking you'd want to keep it in the highest gear possible down to 1000 RPM to avoid engine friction when slowing down. As I said this would be counterproductive if the alternator was going to be used normally, but if it's just going to be used for regen as the OP seemed to indicate, then it might be worth checking out at least.

Why 24V? The car runs on 12V....15V max if you want to keep things fully charged.

Cost is also a factor - $570 is a decent chunk of change. Once you start getting up there in price you might as well have a hybrid that has full regenerative braking and whatnot. Part of the economy of not having a hybrid is to avoid having expensive batteries in the car, whether those are HV batteries or expensive lithium 12v batteries.