What's a slightly rich mixture when 14.7:1 is the most efficient ratio?
How about finer spray at the same duty cycle but higher pressure?
Cars with performance cams and exhaust will lean the mixture out. By how much depends on the cam angles and lift used and the size of the exhaust and type of air filter fitted. You will only know by how much when you dyno the car with A/F ratio readings and graph. If you going that route always use Dynojet as its the most accurate and repeatable.
14.7:1 ratio is only applicable with idle mixture and light load or light throttle mixtures below 3000rpm. Any more load with higher rpm needs more fuel increasing in demand till you get to full throttle high rpm, you will need about 13.2:1 to 12.5:1 depending on engine type, design and manufacturer. Figures quoted are only for N.A. engine turbo engines are different.
Most 100% stock cars unmodified at full throttle dyno run at high rpm usually will show a 12.0-12.5:1 A/F ratio. That is why when you add an open air filter and exhaust you can usually just get away without needing to add more fuel but gain more power. But always check on the dyno because you might be having a car which is tuned just right for stock conditions. That is why always dyno the car first with A/F ratio graph so you know where you stand rather than guessing all the time.
With higher pressure you will only get a marginal or no increase increase in finer spray basically negligible. Most of the time even with standard pressures the fuel is quite well atomized already. Some injectors don't really atomize the fuel at all, it almost comes out like a straight stream as its designed to hit a plate and then the resulting spray atomizes the fuel, usually found in older continental models. In order to get a finer spray you need really high pressure and small duty cycle. Thus DI high pressure fuel systems
So when you increase the fuel pressure even slightly, alot more fuel goes into the engine without affecting spray pattern and atomization. So the slight increase in fuel pressure will not affect the spray pattern.
If you want to do a test. Take out the injectors and get them serviced. On the injector machine you can see the spray pattern. Ask them to increase the pressure by 0.2-0.5kg/cm (usually standards pressure is about 3.0-3.5kg/cm). Then ask them to run the injector you will see little or no difference in the spray pattern but in a timed test you will see flow has increased dramatically. Most injectors and fuel systems are designed to atomize and spray properly at 3-3.5kg/cm pressure, that is why any more pressure will not create a finer spray. But a decrease in pressure will affect the spray pattern and atomization as will occur in a faulty pump or clogged fuel filter.
If life was so simple everybody would just buy a F/P regulator and crank up the pressure. But as I said when you crank up the pressure fuel feed is increased everywhere. So you might get the A/F ratio right at full throttle but idle, compensation, start maps and parts throttle maps will be running rich, so you need to retune all those.
Moreover more and more cars are using returnless systems so its out of the question already to add a F/P regulator.