My opinion on the EFIE is that 100% of the fuel economy gains are due to leaning out the engine. You can run up to about 16.0 AFR before power begins to drop from too lean a mixture.
Every single car off the production line could be setup this way, but the environmentalist hippies have stymied any hopes of this.
Lean mixtures promote higher combustion temps and higher nox emissions. Catalytic converters work best at 14.7 AFR ratios so the leaner mixtures are not compatible with exsiting emission control equipment.
When running in closed loop O2 control, the vehicle ECU will try to maintain 14.7 afr at the exhaust. I own and tweaked an eagle talon back in the day with a piggyback AFR controller. The main purpose was to lean the engine some at full throttle for power gains. However in closed loop mode, the ECU would continually trim the engine back to 14.7.
There was a guy in our group that leaned his car out far enough that the ECU trims maxed out and he was able to hold a lean burn for a considerable stretch of highway. Boosted his mpg from high 20's to high 30's over a couple of hundred miles.
Check out
http://en.wikipedia.org/wiki/Lean_burn