EFI when tuned provides accurate fuel metering over the entire operating range. At low power settings you can set the A/F ratio in the 15-16:1 range for economy and at WOT set it for what the engine wants to make best power - usually somewhere in the 11.8 to 12.5:1 range.
As far as power that the engine would make - well thats depends how much air your moving through the engine - not directly dependant on the EFI system.
