I have a question about them, but I'll get to it at the end.
O2 sensors are used by the engine computer to sense the amount of oxygen needed to completely combust the remaining hydrocarbons in the exhaust stream. Rich exhaust causes oxygen to flow from outside into the exhaust stream, and lean exhaust causes this flow to slow or even stop.
Standard O2 sensors can detect AFRs between 14:1 and 15:1 (much simplified explanation for the sake of brevity), but they don't do a very accurate job of it because they'll go to an equilibrium oxygen flow, which will cause the O2 sensor signal to become slightly inaccurate. Such inaccuracy may be tolerated with post-cat O2 sensors, where their primary job is to monitor the cat's efficiency, but this inaccuracy is unacceptable for AFR control. This is typically why engine computers continuously vary their fuel output between slightly rich and slightly lean, in order to keep the O2 sensor signal from going to equilibrium.
Now, my question is - If an EFIE device is supposed to trick the engine computer into running leaner than before, by modifying the O2 sensor signal to make it appear that the engine's running more richly than it really is, how can you maintain control of the air fuel mixture if the O2 sensor output only goes to 15:1? Because it seems to me that if you're going to go beyond 15:1 (where real fuel economy gains would be realized), you're going to need something other than a device that merely changes the signal coming from a sensor that is limited to 15:1 on the lean end. Otherwise, you have no idea what the AFR is, and you'll eventually end up burning something up inside the engine due to an unanticipated over-lean condition.