The flow of fuel stays pretty linear as rpm's rise and load increases. It doesn't matter if you have 2 injectors running or 4. The same total amount will be needed. You'll just work 2 of them harder. Mazda did it wonderfully on this car. The best power is made by giving the engine only what it needs in terms of airflow and fuel. The runners sequentially come online only as the extra air is needed. This keeps air velocity high. The fuel injectors are also only on in the same runners. When another runner is added, another fuel injector is added to the system at nearly the same time. This is to provide better air/fuel mixing. When the second injector (or third) comes online, the primary injector lowers it's duty cycle to provide less fuel. It needs to balance them out somewhat between injectors. You don't want fuel injectors coming online separately from the intake runners. They work best together and they work best at a certain rpm. Turning on or off injectors during cruise won't change anything and may even hurt fuel atomization.

The rear end gear ratio change is the best option for economy.