It is nothing to do with the amount of injector rails this merely supplies the injectors.
It is the siting of the injectors, with 8 you can optimise for low speed torque, where they will fire at the back of the valve to futher away STRAIGHT down throat where they will have longer to mix being combustion, as said it also helps with latent heat absorbtion.
Advances since 8 injecotr days in the area of ECU makes this(almost) redundant now, as you can get ace fuelling across the board with large injectors as better maps are available.
tabetha