Yes the efi is what regulate the gasoline injection, it measures the amount of oxygen coming out on the exhaust with this sensor called lambda, and control the gasoline injection maintaining to i think is 8% the amount of oxygen that must come out normally, than if for example it senses that less oxygen than 8% came out it will add more gasoline and vice versa...
Yes I think that they are adding only H2 for fooling the lambda, this way you consume more oxygen from the air and so it will automatically sends less gasoline, it is also safer...
However if you want even more efficiency, in my believe you would need to have the highest concentration of oxygen possible, but this would force you to also control the efi signal, otherwise you would only have more power in your engine, witch would also cause a better milage too thought... In most cases is very easy, you just need to use a potentiometer and a couple of resistors to make a voltage divider... The lambda probe will send a voltage signal to control the injection you just need to raise or to lower this voltage signal to actually control it... There are some circuits also out there...
Good luck
Best Regards