Well, I had a look at that EBay listing, and it says better fuel consumption because it gives better fuel efficiency.
However, on the chart it actually admits that the stoichiometric air/fuel ratio is best for economy (as everyone knows). So a lower ratio (i.e. more fuel for the same air) is bound to reduce economy.
Now any company which makes claims that are so obviously untrue would make me back off completely.
I think there's also a lot of technobabble in that ad. Talk about not needing error checking, for instance. As far as I can see all the thing does is reduce the voltage out of the air sensor, so where would error checking apply?
To my mind it's a higher class scam than the cheapo resistors - I'd want to see before-and-after dyno records (not the butt dyno!), and fuel consumption records before giving it any credence.